HELP no sound on left side
 

hi every one new to this site.

I got my bmw x5 2002 last sunday

I am trying to wire a subwoofer in to it but no joy so far :[ then I seen only driver side door speaker works
I have been looking at this site and see a few people have had this problem.
people are saying this has some think to do with the amp.
can any one tell me what to do next
that would be great :]

p.s I heard it came on once when I was driving and went back of again.