2003 Cartier here with similar issues.
TL;DR - I have a bad DDM or the wiring in the door (or one of the things connected to the DDM, if they can cause this problem) is damaged somehow and is causing the DDM to act like it's gone bad. with all the associated symptoms (and no blown fuses). My mechanic thinks it's the wiring by the door from opening/closing, and it's even more likely than usual since the wiring has previously had issues before and there are some visible repairs, heat shrink tubing and all, and I could've damaged the wires by spraying water near those areas. I'd probably just buy a used DDM first, that I could easily install myself, if I knew that the same model number DDM is all that is needed to ensure it gets along with the rest of my car (as opposed to same model and/or trim level and/or features, etc.) and if I could (practically?) rule out wiring without it being as crazy difficult and easy to screw up as it appears to be.
END TL;DR
It started months ago - key fobs stopped working, tried to reprogram them from scratch, no luck, couldn't be sure that anything I bought online would work or work for very long.
Gave up on that.
The rest happened almost all at once in the span of a few days or so.
The side view mirror controls stopped, the trunk release stopped working, the power door locks stopped working, the keyless entry pad stopped working, the power seats stopped working last, and then after the "easy entry" was able to signal to the seat to go all the way back and with no way for me to stop it, now I can barely reach the pedals (particularly since the seat was set back an additional several inches at the factory for the quite tall original owner).
After trying to replace any relevant fuses (that looked fine anyway) and then using a multimeter to test that they are not bad, and looking into it some more I noticed the commonality.
Everything points towards something to do with the Driver Door Module and I know how to get to it easily, it's exact model number, even found a few I could buy, but my mechanic (really great guy, loves my car, been working on Fords for decades originally as part of a Ford dealership) thinks it might be the wires by the door from opening and closing it over the years. I had called to ask if he can program the DDM if it turns out to be that, but he said he'd have to order it to come programmed and from what I gathered that could be over 10x as expensive as a used part, that I would prefer to go with, assuming that the part works on my car, if I knew that it would.
It seems like the specific part model number varies a lot (by year only? by trim level? by specific features too?) but I don't know if that alone is enough to know it would be programmed to work on my car. My searches have come up with inconclusive information. I don't know if I would need to get a specific model number DDM from a car with all the same features I'd need it for (as some have suggested), or if a range of model numbers from nearby (car) model years would work but it also needs the exact same features that would need to be determined another way, or if simply the correct part model number would be enough. None of the local salvage yards seem to have an 03 Cartier, but I think there was an 04 Ultimate as about the closest
On top of that, there didn't seem to be any mention beyond that I'd need to record the new keyless entry code. Would it also affect the key fobs? Would that all work nicely with the rest of the car and allow me to program the key fobs from scratch, or is the transmitter code stored somewhere else in a computer waiting for a working DDM to ask for it and then as soon as I get one that works, those key fobs will start working again like nothing happened and I can create more whenever I want to off of them?
If I knew this information, that it would be a sub-$100 part that's easy to replace, only need to match the model number of the DDM, and it wouldn't cause me any difficulties beyond having to relearn the new keyless entry code, then I wouldn't hesitate to buy one now to try first.
But the potential wiring problem seems very possible as well, maybe even more so based on other details, but I couldn't say what is more likely and I can't seem to diagnose it myself even though I would like to - in theory.
I got some wire repair tools, watched some DIY videos, and tried to learn how to locate where a short or open circuit might be located from (still don't really know how I would, especially with only a multimeter, not a test light or those signal/receiver things), and when I got a better look at all the wires coming into that door again, I was completely overwhelmed and have no idea where I would even start.
I had replaced the driver side mirror myself when it was cracked, and disassembled that panel to get to it. It's relatively easy to get to the wire harnesses and connectors that clip directly into the buttons on the driver door panel, which just pops out, but tracing them a short distance leads to a giant tightly wrapped bundle of utter confusion.
I spent a lot of time trying to find wiring diagrams, but even if I did have them, I'm not sure it would help me much in making sense of physically locating each wire (removing the door? unwrapping the electrical tape? tracing wires individually from their outputs?). Would it also give me the information I needed to know if enough current was running through them, or the right voltage was sending the proper signal to the various control modules? I once had minor knowledge about circuits but practically little experience.
Regardless of if I could figure it out, if it's in or near the accordion boot, I fear making a gigantic mess that I can't then undo or further damaging wires while trying to repair them or leaving them situated in a way that during normal use they will get stretched or rubbed on and then fail prematurely because of my attempt. However, this time, I did notice several heat-shrink tubes where wires in the door panel itself (before they split off into a handful of large bundles instead of one gigantic one) must have already been spliced and repaired, although I'm not sure when that was done or why.
I think I might have had issues with the windows before - and months ago, while the driver side window goes down easily, unless I push it quite hard and at just the right angle, it won't go back up.
On top of that evidence that the module itself might be okay but the wiring itself isn't, I had issues with wasps making nests behind the mirror itself and in cracks and crevices along the door frames and pillars since I wasn't driving as much due to lockdown and so they made themselves comfortable, and I tried to flush them out at night with water somewhat carefully but I could have flooded the side view mirror in the process or inadvertently splashed enough onto the door itself that it misted or dripped into the panel when I had the door open to get in between. I got rid of most of them, but they came back pretty quickly and eventually I sprayed them with pesticide (which comes out like a foamy liquid) and then, so that me and my cats (who like to rest on or under the car) would be minimally exposed to those chemicals, I washed off the same areas with water to remove and dilute it.
Is it possible that there could be some sort of downstream effect from the side view mirror wiring being damaged by that? Could a short or open circuit caused by corrosion or other damage to the wiring cause some sort of signal (or lack thereof) or feedback that prevents the DDM from working without destroying it outright, that could be resolved by fixing said wire or wires?
In the meantime, I may need to drive the car a bit more, and I've resorted to using a couch cushion like a booster seat, while the seat itself is all the way back and, as well, leaned back much farther than I prefer. It's not particularly safe for me should I be in an accident, and I am probably more likely to be in one given how uncomfortable and unfamiliar the positioning is and how much less support I have in that position from my seat.
I think I'll make another post asking for help with that as well, since I tried fruitlessly for hours to find out how I could move the seat forward, and the closest I've gotten at this point is, remove the seat, then do something, and merely removing the seat might be literally impossible for me without doing something more drastic than I have tried.
