Ground fuse? There would be absoulutely no reason to do that-I have NEVER seen one installed at any level. Your scenario actually sounds like the car was jumped and cables were connected in reverse. I have also seen that type thing happen when a car is hit by lightning (lightning does funny things to mediocre grounds).

Diesel products? Maybe one step above flea-market specials (ie: Boss, Pyramid, etc)--MAYBE. Even one of my main electronics distributors steers all his accounts away from their products. He told me a while back that the DOA ratio is almost 30% (meaning 3 out of 10 Diesel products are returned defective out of the box)!

You may be better off purchasing a used amp from a reputable TR member or reputable pawn shop if that is all you can afford. Used amps are usually trouble though (there was a reason that it is no longer in someones car...)