I had the same problem too. The original owner had put on a towing package and didn't ground the wiring right and we had lost all of our tail lights going to the brakes and electric systems to the trailer. I would look up on the driver side under the bumper or right next to the gas tank and see if they wired it for a system to pull something first. If that is not wired in then check all bulbs in the tail lights and you will find something blown there. Even a turn signal will make everything go nuts on these GM vehicles. Good luck. If you need help just email me I can walk you through this wiring but im still lost why my car only goes 1/2 an inch to try to start and wont move after that got any ideas?