OK, long story short... 2008 Signature L with about 30k miles... my remote keyless entry stopped working...reprogrammed it, it forgot it again, tried to reprogram the next day and it would not program with either remote. Suspected DDM. Read mainly this thread but a few others... I gathered from the Ford part sales pages that any 2008 unit from a Ford CV, Mercury GM, or Lincoln Town Car should work as they list one module for all vehicles.
Opened my door up and found the DDM part # to be 8W7T-13C791-AD
Went to the local pick a part and found a 2008 Ford CV w/police package pulled the panel and low and behold the exact same part #!!
Brought it home and installed.
As expected some things did not work such as power seats, memory functions, heated seats, etc..
I had previously read the programming from the other module using Forscan.
Wrote the previous file to the new (old) module and cycled the key as instructed.
Everything seems to work!! Seats, memory, mirrors, seat heat, etc...
Now I hope it continues to work

If not rest assured I will update.
This is a for sure Forscan program success as the unit did not function before and does now so it was a field program of the DDM, it even wrote the factory door keypad code and seat memory settings to the unit so I didn't have to mess with that.