Front twitters - no sound is coming out
 

Hello,

I've bought C70 2001 and it turned out that the front twitters won't work at all. I checked if there are present and they are, wired but no sound is coming out, nothing.

My car is equiped with SC-901 radio and prologic sound system with the amplifier in the trunk.

Any idea what may be wrong and how to repair it very appreciated.