Need help with replacement stereo

I decided to put my CD player from my other truck in my '90 4Runner. I couldn't find a harness at Wally-World or AutoZone, so I cut and spliced. The stereo works, but I have no sound to the rear speakers - none at all. Here's the scenario: Toyota factory system (Matsushita AM/FM/Cassette) has the connections for both front and rear speakers. The replacement has only front. I tied them together (LF with LR, RF with RR). I also have an amp that was used for the rear speakers only with the original system. So, for each side, the wires come from the stereo to the speaker wires - the front go to the speaker and the rear go to the amp, then to the rear speakers. I tried diconnection the front speakers and just connecting the rear with the same result - no sound. The amp's fuse is good and it's powered. I also checked the resistance for front versus rear. The front was 43.5-44 ohms and the rear was 48-49 ohms. I attribute the difference to the amp and its connections. I was very careful when making the cuts and splices, marking every wire before they were cut. Anyone have any ideas? Also, what's WHITE w/ GREEN (#5) on the rear speaker harness used for? I can't find it documented anywhere, but it's there.