You are probably running into one of two problems, with regard to the volume control problem:
1) You are connecting the Clie to your stereo's amplified input RCA plugs. This would bypass the stereo's amplifier.
2) Your stereo allows you to select amplified or "line" for the RCA inputs. In this case, you are connecting to the correct plugs, but have it set as amplified instead of line. Using the line setting, the stereo passes the signal through the amplifier, and you get loud sounds and the ability to adjust the volume. (Not all stereos allow you to switch the inputs between line and amplified.)As for sound in only rear speakers... In most high quality systems, you have seperate connections for all four channels (front left, front right, rear left and rear right.) That way you can set up amps in different ways to get the results you desire. You probably took the easiest route by using a splitter to get audio to all 4 speakers.
If you need to, you can always get a cheap car amp to run you clie (or other aux) input through before sending it to your stereo. That is, if you can't change the input setting to line.
BTW, if you don't still have the manuals on your stereo, Pioneer's website has complete manuals for almost all their car models. I also have a Pioneer (DEH-P9200R), and it's awesome. (And it better be, considering what I paid for it! $$$)
Good luck,
Tip DS