My girlfriend's stepdad has a 2000 GMC Jimmy that he bought from a family friend before that friend passed away.
At some point the inside driver side door panel came off for some reason and he couldn't get it back on, so he just left the electrical switches hanging by some string inside the door.
He stopped driving it regularly a few years ago and it's mostly sat around, but it *can* start and drive. It just had a dead battery when he tried to run it at one point. He just figured the cold killed it during winter. He got a new one and then later, the battery was dead again.

I don't really know much of anything, but from what little I've learned, I'm guessing it might be parasitic drain from a short in the door panel wires.

Is it expensive to get electrical wiring checked out and fixed? Is there other likely causes?