Rca input problem

ist not under warranty anymore, but i did take it apart and all the connections were fine, but i know its not the cable cause both of them work on the on good jack

It's possible you have a problem on the output side as well.  Have you also tried switching speaker leads at the amp to verify your problem?

what do you mean?

As you first stated, you've eliminated the RCA's and the head unit as the possible problem. 

The other possibilities could be the amp, the wiring of the speakers, or the speakers themselves.  My point was that this doesn't mean it's the input side of the amp with the problem- it could also be the output side as well.

Here's something else to check before you write the problem off as the amp itself:  Switch the speaker wires at the amp.  Hook up the speaker that isn't playing up to where the speaker that is playing is connected, and vice versa. 

Now if the problem switches to the other speaker, then the problem is definitely amp related.  However, if the problem stays with the original speaker, then it's in the wiring, or the speaker itself.  Time to pull out the multimeter and take some more measurements.  It's possible the connections have come loose at the speaker, or you've blown a voice coil.