Good questions... I will try to answer for you. Others will help out if I leave something out or have incorrect info...
First off, I have learned that anything is possible with car audio. The people you talked to took the easy way out. Technically they are correct, but the issues could possibly be overcome. Here is why:
1.) Most CD changers (like your RF *actually Clarion I think*) correspond with a same brand head unit. The head unit has to be able to communicate with the changer. For example... I have a Kenwood head unit and it has a preout for a CD changer, but only Kenwood CD changers will work with it. Both units have to have the same "Software" (for lack of a better term) to communicate with each other. If you are familiar with computers, it would be like putting a Macintosh chip onto a motherboard designed for an Intel chip... they just don't work, or communicate. So you will have to find another Head Unit that speaks the same language as your CD Changer... then you have to figure out how to adapt the wires and plugs to make it work. Most companies use different style and sized plug adaptors. It may be possible... but will be very complicated and difficult I think.
Check this link if you are really serious about it....
http://www.b15sentra.net/forums/show...ght=CD+changer
Massredspecv seems to have figured it out??? Perhaps you could ask him?
I would hold this point against the guys you talked to... for all practical reasons they are correct. It is possible, but very difficult and complicated.
2.) Speakers... This one i would be mad about... If they had an ounce of brains they could figure this one out. Or they could have at least explained it to you. Here's the deal...
Your RF system is amplified. The amp is in the trunk. Your deck is not amplified becuase of that. So the system is set up to have the head unit send a signal via 18 or 16 gauge speaker wire to the amp. At which point, the signal is amplified and sent from the amp to the speakers. If you were to install a new head unit, you would find that there are no direct wires form the dash wire harness to the speakers. You would also find that the pre-amp output of your new head unit would be in the form of RCA size cables, and your RF amp does not have RCA style inputs. So technically those guys were right, you would have no way of getting an audio signal to your speakers.
But, with a little extra energy it can be done. You would need to do one of a couple different things.
1.) run new speaker wire from the head unit to each speaker (thus eliminating your RF amp.)
2.) Get rid of your RF amp and get a new one.
3.) Buy line level converters, or line outs, or whatever people call them.. They are little units that allow an audio signal to convert from an RCA cable to a typical speaker wire..
Here is another really good link that will help out.
http://www.b15sentra.net/forums/show...ght=CD+changer
Remember anything possible. But not everything is worth the hassle to do.
Good luck... let me know if you have more questions. Hope I have helped.
GB