It sounds like your DDM is going.
Honestly, I'd take it to a dealer and pay for the diagnostic on it. Their scanners (tech2) can read and see things that we can only guess at without it. Rather than chase down a bunch of "maybe" items, they can tell you what exactly is wrong. Then you can do the legwork replacing the problem items, so you don't pay them for labor.
If you have heated seats that work, try turning on the passenger side heater and see if it works. It sends a signal through to the DDM to turn it on. If it worked before, and now doesn't, then even better bet that your DDM is going.
Its really too bad you can't get a tech2 for under 5g. It really makes it tough to diagnose communication and module problems in cars this complicated.
2004 Buick LeSabre Custom FE5 GXP suspension conversion. 170Kmi
2001 Silverado 4WD 5.3L, 33" BFG ATs, Z71/Z82 pkg, 147kmi.