'93 Toyota

Did u check the underside of the rear deck?  Also, when you match up the install harness with the factory plug, and look at the factory harness, does it even have wires for the rear speakers?  Finally, I'm assuming since you said it's a 2-channel radio that you meant that the car only came with two speakers (fronts).

I've found almost always that they use the same wiring throughout the car regardless of whether or not a particular option is installed.  I've seen many vehicles that had the "no radio" option but that have all 4 sets of speaker wires run to their proper locations, as well as a harness somewhere in the dash that is specifically for a radio.  My advice would be to make sure the factory plug does indeed have those rear speaker wires, and assuming it does, just keep on looking in the back of the car til you find them.