When I got this car the dome/map lights weren't working, but when I went to take the thing off, a bolt or something broke off and got lost before I even had a chance to look at it. Now I'm completely lost on how this thing goes back together.

I assumed that that the positive wire bolted through the unit to the inside where that little loop is hat is connected to two other wires, but whenever I do that it grounds out the whole unit and I get sparks when I try to screw the thing back in place.

So, if anyone knows how this thing is supposed to go back together I'd really appreciate the help.

Some pics for reference:

 

Its likely the reverse. Usually, ground will bolt to the chassis.

 

Yep, thats a ground, and you didn't break it. The screw for the dome light holds it in place and provides the ground.