Why wont my subs work in a bronco?

My friend has a bronco and I have tried to install subs to it and for a while they worked alright. They always had a problem where if they were up to loud they would just shut off after a while but other than that they were ok. one of the fuses blew and we replaced the fuse and everytime it was hooked back up the amp smokes a ton. Why is this? Is there anything that I can do to fix this?