wiring trouble warning n00b

ok so here's my situation... Just got this lexus, has a pioneer head unit, two tens, aftermarket tweeters and front door speakers, no rear speakers. Everything is all fine and dandy, sounds nice, but not having rear speakers is bugging me, so I wanted to add some. Anyways rip apart the interior, notice some things that I'm not familiar with. the front door speakers (hooked to the amp) are hooked up to the MID/REAR RCA plugs on the head unit. The grey and white wires on the deck harness (go to the REAR speakers I think) don't have anything wired to them. The greey and purple pair does though, I'm thinking it's the tweeters (haven't removed the door panel yet though). I want to know how to add the rears in, they wont' be powered by an amp, just by the headunit. I tried to hook the rears up to the white and grey pair but no sound came out fo them, I'm assuming that is because something is hooked up to the mid/rear RCAs? Anyone got any advice for me?

Tahnks for the response, it's actually a , forgive me if I miss a number, pioneer premier deh-960mp or something like that, it folds down to reveal a pretty screen. Assuming it doesn't have the pro standard thing, would it be ok to.. hook the front speakers to the FRONT rcas, wire my rear speakres to the rear wires on the wiring harness..then..idunno about the tweeters. Woudl those go in line with my front speakers that are hooked to the amp?

OK sorry for all the replies, on the bottom of the deck (and int he manual) it has two different modes, standard and network network dedicating speakers to high low and mids, and standard being whatever. So it is on the network setting right now. My tweeters and door speakrs both go to a crossover in the door, then to the amp and from the amp to the deck into the MID RCA plugs. When I try to plug speakers into the head unit wiring i can either not hear anythign or barely hear it. I want to get these back speakers in, what if I just switch the switch to standard with the front speakers wired up to the amp in hooked to the rca?