I have an 04 GXP Bonneville. Great car so far. Almost 100,000 and it is running great except for the serviced stability system warning is on all the time and the "trac off" light stay ON and theABS light stays on as well. It has been diagnosed twice. Once I was told that it needed a rear wheel bearing because the sensors were bad. The second time I was told the sensors were bad but that I also needed the wiring harness that controls the speed sensor. One person said it look as though the sensor was exposed and had some rust. Surely there is a better way to actually fix the problem that does not require over 600dollars in new parts. any ideas?? thanks.

Welcome to the site.

That car should have body codes telling exactly what sensor the problem is. My wifes montana said RF WSS, so fixing the problem was easy. A basic code reader may not give this code, you would need a code reader with enhanced codes.

And as I always mention in these type of problems, it can be as simple as connecter corrosion. First thing to try is pull each WSS connector apart and grease them. Not always a miracle cure but has stop the problem for some of us including me.

yeah as John mentioned, its a common problem with the GXP'*.

Some grease and you might be off the hook

I'm dealing with the same problem right now on my 2005 w/130,000 miles; service stability message, trac light, abs light.
I had the codes read for free. Looks like the issue is the LF sensor.
I have a new bearing/sensor ready to install. This will be my second one.

I've replaced my wifes 2 times now. And again she'* getting TC light. Starting to get old

One other item it can be and this item does not trip the codes. You could have a small hole in your rear shock hoses coming from the compressor. If the rear shocks don't remain inflated to full capacity then the SSS light will come on. If the shock is getting very little air pressure it will also cause the TC light to stay on.
Best way to check for these small holes is to jack up each side of the car from the rear indivudually. Once you have the rear jacked up follow the hole from the shock to the spot in the wheel well where it comes out from under the plastic. Have someone turn the key to the on position without starting the car, and the compressor will kick on. If you hear air in the area of the wheel well where the hose enters then you have a leak.
These hoses are made of plastic and when they rub on plastic(bad design by GM) they tend to melt and a small hole will allow air to escape the hose. Your shock will still be getting some air, but if it isn't fully inflated then the SSS will come on.
I have found that between the dealers and the local repair guys they miss this item more then any other.