bypassing rear nissan amp...help

Hi.......I need help on how to bypass my Clarion 'active control' amp in the rear deck of a 1992 Maxima...I replaced the 'active control' cassette radio with a 1997 Nissan CD/Cassette combination, I'm pretty sure this radio has an integrated amp.  Right now I get a constant hiss from the rear speakers and I think it's because the factory amp is being powered by additional power from the radio.  At least that what the stereo shop says.  If this would be easy to do, I'd like to do it myself.  I've already paid the stereo shop to first wire the radio with a harness that would plug into my factory 1992 harness (65.00 later the rear speakers didn't work and the when I turned the lights on the stereo went dark but no hiss) now 70.00 later all speakers work but terrible hiss but now the deck lights stay on but no dimmer.  So if I can figure this out myself I'd like to give it a try and save another 45 dollars.  The factory rear speakers are 4 ohm but I do have some new Boston Acoustics that I can install if that would sound better.  The door speakers have been replaced with pioneer speakers the original ones were bad.  Does someone make a adapter that can be plugged into wires before they go into the Clarion amp? Or can I just spice the wires before they go into the Clarion amp?

Thanks for all your help.