Check the charging system. The generator is a very high failure item on GM vehicles since the '87 model year. Measure the battery voltage with the engine running. It must be between 13.75 and 14.75 volts. If it is okay, have it professionally load-tested on the vehicle. If you can only get one third of its rated output, and "ripple" is high, it has a defective internal diode and the unit must be replaced. To reduce the number of repeat failures, replace the battery at the same time unless it is less than about two years old. As they age they lose their ability to dampen and absorb the huge voltage spikes these generators produce. It's common to go through four to six of them in the life of the vehicle.