Electrical problem!

Hey, what's up guys!? Well, I've gotta a problem here and I really need your help. The thing is that one of my car's rear windows (it's a 1990 vert) stopped working some time ago. So I took the motor out thinking it was broken or something and got a new one. I installed the new one and the thing won't work. So I went back to the shop thinking the motor was deffective but we tested it and it was OK. Now, I'm sure there's no power going to the back. What can I do? Which wires should I check or what? Yes, it's winter and I'm not putting the top down, but I want to have my car working at its best!

Thanks a lot for your time!