Since the jerks in question may have done anything, and their approach was sloppy enough to blow a fuse, you will be best advised to examine everything about the setup:
- wiring to the radio itself. Be sure it is plugged in, and no wires broken nor crossed.
- fuses. All of them, even the ones in the car's fuse-box (but, do not pull the fuse for the radio itself. That would make for even more difficulties with getting this system fully working again.)
- wiring from the radio to the front door speakers. And to the rear door speakers. In detail: look for breaks, crossed wires, disconnections and cuts, etc. It might be that the front and rear door speakers were also powered from the accessory amp ? which would explain their not working, undoubtedly, if that were the case.
- and, make sure the door speakers are selected on the radio itself. It may have the external amp selected, that is no longer there.