I did this in my younger years and the cars I got always had some hidden fucked up problem.

My minivan is (well, was) like this. The person I bought it from said the roof-mounted DVD player was professionally installed. I got it home, pulled the interior apart (I was installing a new headunit, backup camera, and some other things), and found the “professionally installed” wiring crossed in front of the side curtain airbag. It wasn’t secured anywhere at all, either. Just flopping around. I fixed that up along with some other issues I found.