six channel from four channel head unit

I'm sure this is a common question, but I have'nt found what I need in the archives.I have a Blaupunkt San Jose in my Volvo 740 Wagon. I am installing a four channel ADS amp along with a two channel amp, (Also ADS). There are 2 dash mount 3 1/2" speakers, a set of 4" front door mount and a set of 5 1/4" rear door mount speakers. For the purpose of clarity I will not discuss the sub. I am trying to figure out a way to power the six channels of sound from a basic front / rear fader. I'm considering a "y" of the front channel RCA into one of the power amps.                                                                                                                                                                Is there any such thing as a simple volume control that I could connect between the "y" and the power amp? What would be ideal is a device that had stereo RCA ins and outs. I'm not having any luck finding this, possibly because I'm not asking the right questions.  Any help would be greatly appreciated.

well how many sets of RCA outs are on that head unit?  i think there is a front and a rear output.  if there is then do this:  rear output rcas go to the 4 channel amp running the 4" and 5 1/4" speakers.  you need two "y"'s for this.  and the front ouput just run straight to the two channel amp for the 3 1/2" speakers.  got it?  if your confused just write back ill try to explain better.

Thanks Jimmy, and that does make sense. What I'm looking for (using your configuration) is some sort of level control over the two sets of rear speakers.