1992 Cadillac Seville which grounds does one check for back

My car just received a tuneup 2 days ago. Prior to the tuneup it would backfire and lose power at supposed random times. Mechanic said the tuneup would correct that deficiency. Day after tune up car loses power and shuts down while driving. Would not start because lack of spark had coil and icm replaced. Mechanic said this should help. Car started up. But as soon as I turned on the headlights I loss power and the car started to backfire. Determined that the car acts up when the volts drop below 12.8. Mechanic said it sounded like the car was running out of gas. But there was plenty of gas in the car. That was yesterday. Today the car loses power regardless if the light s are on or not. Someone suggested the alternator another daid the ground were faulty/and or dirty in there car after they replaced the alternator the had the grounds inspected and that is what the dealership. How do I determine the grounds that are bad. Or where do I locate them. If you have any other suggestions I am more than willing any suggestion. I am tired of throwing money at this car