Testing Amp channels

i'm really not totaly clear on what you wrote. but what i think you said is that your using factory wiring for the output of the amp to the factory door speakers and you have one set of rca's from hu to amp input. but the amp has two inputs. there should be a switch for 2 channel and 4 channel operation on the amp. if it does switch it to 2 channel op. and you will internaly supply input to the other two channels but will lose the option of fade but will still have balance. if it does not you will either need the get a hu with front and rear output or get a y-adapter to get one male rca into two male rca. do that for both channels that you have and you get 4 channels of output.

if you want to check the operation of the rear channels use the front rcas and put the into the rear channels of the amp. you know that is a good source of audio and if the rear speaker dont work you most likly have a problem with the amp or there after.

a converter is diffently a good option. i forgot to list that thank you for the correction. but if he switches the front speakers to the rear speaker channels he still has the problem of no audio supply to the rear channels. try the switch first and if it dont have one switch the rcas from the front input of the amp to the rear and you will have that good audio supply and if that work , then you can use one of the options list previously. convertor,hu with four channel output or y-adapter. the first two options are the best because you will have more control on the audio system fade and balance. 

if you do this and still don't have sound you will need to check the intgraty of the rear speaker curcuit. by taking the rear speaker wires and hooking them up to the front channel after disconnecting the fronts of course. because you know for sure that them two channels works. if so at that point then you can say that amp is got to go back.

Fingaz22 and stevdart are both leading you in the right direction.  You know that you have good signal going into the amp since you have sound out of the fronts. 

Take the RCA's out of the front inputs and plug them into the rear inputs.  Do you have sound out of the rears?  If so, put in a set of Y-adapters and call it good.

If you do not, put the RCA's back, and switch the speakers leads front to rear at the amp.  Do you have sound out of the rears now?  If you do, the amp is bad. 

If you still have no sound out of the rears, disconnect them, and try to ohm them out with your meter.  Also check to see if there's a lead shorted to ground.  If you get no continuity, then you have a problem in the speaker wiring and/or the speakers themselves. 

Try putting the RCA's into the rear inputs, and connecting the front speakers to the rear outputs.  If you get sound out of the fronts, the amp is good, and your problem is in the wiring and/or the speakers themselves.  How are they wired?  Did you run wires directly to them from the amp, or did you run them behind the deck?  Try running directly to the speakers, and check your connections.  If you still get nothing, and you tested the wire for continuity (to ensure there's no break in it), then the speakers themselves are bad.