2006 vue redline sound problems

I got a 2006 Saturn Vue Redline. I did have my system hooked up to the factory radio, (which is a Panasonic Automotive, Model No 15850682) and of course the bass drowned out the vocals. I learned then that this car is equipped with a factory amplifier bolted under the passenger side back seat. Well, I ordered a radio harness and mounting kit for my car, and installed my Pioneer DEH - P6800MP radio. Now the harness only had a 12volt wire, ground wire, remote wire(for factory amp I suppose) and a dimmer wire. It came with no speaker wires to connect to the radio, assuming because it hooks up to the factory radio and from personal experience and being a noob and learning, twisting amplified wires to radio will blow the deck. Back to the point, the harness only had RCA cables and this was new to me, but I went ahead and plugged them into my deck. I do hear my vocals, or highs whichever you want to call it, but they are MUCH more quieter than they were on the factory radio and listening to music is kind of pointless because all I am hearing is bass, BOOM BOOM, barely hearing the voice unless I turn the bass down. My question is, is there anyway you can adjust/turn up a factory amp? Or do I just need to do away with it and install an aftermarket amp for the door speakers? Putting a aftermarket amp in is the only thing I can think of doing unless you can turn the factory amp up, and I would sure hate to have to do it because then I'll have to mess with the door panels to get to the speakers and run new wires.
I figured with it being a newer vehicle from my previous car I wouldn't have that much problem with my door speakers but they are just so quiet. Any suggestions? thanks

It is common practice to have to cut the RCA connectors off the harness and wire the deck's speaker wires directly to the wires in which the jacks were cut from. 

I am not sure about your vehicle, maybe someone else can confirm.

Yeah, thanks man I appreciate your help ... It's a scary procedure to me, I feel like I might smoke my Pioneer lol. I just found a crossover laying around and I didn't think about it. I've never used one nor even really know what it's for but could that work? to turn the highs/vocals up?

It could possibly give you a small amount of gain.  But if the amp is looking for speaker level in, and you supply it with slightly boosted pre-amp level signal, it will still not be enough.

Thanks, "i am an idiot" I cut the RCA plugs off and tied them into the radio's speaker wires. That worked well, I am satisfied it fixxed my problem! :)

I am glad we could help.