I Have 1999 Volvo C70, and i was normally driving when my lower door speakers just stopped working so where i could search the issue, Back basses Works normally and everything else, and i know that amp is in the trunk but i Have convertible so there is some mechanism of roof that is in the way so if there is anything else where this problem can Be i would Look it first.... Sorry very Bad english....

 

Issue is either wiring or the amp. You may need to open up the dash to access the amp then run a test signal to see if the speakers work. If they are good then its time to check the amp.