Sorry to hear about your problems. This is exactly the reasons I try to do my own work. If I can't fix something, then I would let someone else do it.
For "aftermarket" work, I would stay and watch the installation. It's a hassel but I've noticed if you're around watching them work, they'll take that extra step to do things right.
If I were you, I'd take the car back immediately and talk to the manager or owner. Don't take "NO" for an answer. Worst case scenerio take them to small claims court.
BTW, I had a friend who owned a tinting / customs shop. He used to hire HS kids or kids just out of HS. They were somewhat experienced with installing various things, but NONE of them were certified. He hired these kids because they were cheaper to keep on staff.
GL
jrmd01@yahoo.com