Audio Systems: Car Radio Problem, radio problem, radio out

Question
I am trying to fix a 1998 volkswagon jetta factory cassette radio for a friend of mine.  She said that someone put her car battery in backwards and it messed blew her radio out.  I took the radio out and replaced the 10A fuse in the back of the radio.  When I tried to hook the radio back up the 10A fuse was immediately blown even with the car off.  I didn't know if something is wrong with the radio like it's got a short in it now or something with the car.  I was thinking it was something with the radio because the car works fine and the radio must be getting power for it to blow the fuse on the radio itself.  

Answer
there is a device called a diode that is soldered in place across the power leads internally that has shorted, like it's supposed to do in case it gets hooked up wrong. this is to protect the rest of the internal electronics. it's not hard to find, but if you're not 'soldering savvy' i strongly recommend letting someone else do it.