99 Deville Ghost

I recently agreed to work on mother in laws car. She said it would backfire, and trunk and fuel door open. I agreed to see what I could do.I used car for several days and problem never happened to me. After visiting with my mechanic friends (they thought I was nuts) they offered no clue. I visited with a friendly dealer mechanic and he suggested crank sensors to cure the backfiring because they controll start and run timing. She agreed to go ahead with the cost. (not fun to get to.) With that done and few other maintenance items, battery disconnected for a period of time I was ready to take it home. I turn key on to start engine, no backfire but trunk pops open and fuel door opens. Seems that little Ghost still resides in the car someplace. Any help would be appreciated.

Son in law (Ghost hunter)

I turn key on to start engine, no backfire but trunk pops open and fuel door opens.

This can be a symptom of low battery voltage. Makes no sense to me but it has been reported regularly. Verify integrity of connections, battery condition and charge level.