Non stop pinging sound car on or off and backup lights on issue

[Hercar]

My wife's 04 LS V8 Sport started a new issue last week. There is a Non stop pinging sound car on or off and backup lights stay on all the time issue. Code reader says 0 faults. The beep beep beep is not the same as the door or seat belt chime. It sounds to me like it comes from the rear deck behind the back seat. I pulled the speaker wires and it still has the pinging. I disconnect the battery and reconnected. Same bit. In the relay box next to the battery if I pull the second relay from the right it stops. I tried switching relays but it does that with all of them. For some reason the other day I decided to unplug the the electric trunk release and that stopped the ping and the back-up lights went back to normal operation; just had to use the key to open the trunk. The next day I decided to wash out the trunk release with electric contact cleaner, then lube it, plugged in the release wire and all was fine. Today back to the same crap but unplugging the trunk release wire did not stop it. I had to take out the backup light bulbs to turn them off to save the battery (2 months old). It has sat in the garage beep beep beeping for hours now. Garage kept, have owned it 20 years, 79,000 miles now, never driven in the winter so it has never seen salt. Any ideas?