Same problem

If your head unit has a separate setting for a non-fader then make sure that that is turned up and check the gains on the amp if the gain is all the way down you wont get any output. And if the gains are down only turn it about 3/4 of the way up, never all the way. If you are getting signal from both channels but it is faint, it is most likly not the RCAs or the RCA jacks. I would say it is a gain issue or a bad amp. Let me know how it goes. Good luck.
-Andrew Hawkins