Cadillac: 1990 fleetwood backfires, loose wires, ground wires

Question
My Fleetwood idles rough and when driving will backfire sometimes. It now has a hole in the muffler from backfiring. My friend said that she saw a huge spark fly out of the muffler as she followed me home. Now this car has been to 2 shops and is still acting like this. I have had it tuned up, replaced the fuel pump and fuel injectors, the ignition control module, etc but it was no help. It will run alright sometimes and then all of a sudden it just start's to run real rough and then backfire's. I noticed the other day that it runs better when you don't use any of the power such as headlights even if you press on the brake pedal it will start to sputter and run rough but when you turn them off it will stop. I am at the end of my rope and wallet with this car. Do you have any advice?

Answer
1. have the alternator tested for proper voltage and amperage output.
2. Check your headlight ground wires.
3. Look for loose wires where previous work has been performed lately.
4. Have the ignition timing tested in both PARK and DRIVE to make sure that the ignition module is advancing the ignition timing correctly.