I have 2 1999 STS with the same problem. Both were perfectly running cars but after changing the radio to an aftermarket one or putting in an alarm multiple electrical problems have occured. They both started off with random dash messages such as "service ABS, service air bag,service suspension, ect..." and soon progressed to overvoltage problems where the car will not regulate the voltage coming from the alternator. The car would normally run at 14.5volts but on acceleration would reach 16.5volts. I bought my car like this from a friend of mine and have currently switched everything back to stock to the problem but it still persists. Now have a customer who is experienceing the same issue under the same circumstances and would like to help him and myself out. Has this ever happened to anybody here and if it has what did you do to remedy the issue? Any help will be much apprieciated

Your problems have happened to a lot of people who messed with a Cadillac wiring system: Most of the wiring carries not only voltage and ground but is also a 1 kHz serial data bus between modules, sensors, and the PCM. Tap into the wrong one somewhere and the data stream is interrupted and quite frequently removing the aftermarket electronics won't cure the problem because the module is now in a fault condition. The only way to clear the defaults is to systematically troubleshoot the wiring /modules with a Tech II GM Diagnostic tool.

Read through this forum and Audio/Security/..... for some real horror stories.

Go to Audio/Security and track down STS2003. He's pretty good at getting things to talk to Cadillac wiring systems.

I just noticed that if I turn the car off, wait for the car to shut the interior lights off, then open the drivers door the gauges move like when you turn the key to the accessory position. Is this what normally happens on a 1999 STS or is this another symptom of the problem cus it only does it when the drivers door opens up and not the other doors?