I think you may be SOL duuuuude.
I do have my old OEM unit, and it's probably the same as yours (basic: radio/cassette/singleCD) . I just checked it and the back of it has no RCA's and no CD Changer input either. Just square receptacles for the harnesses and the 2 antennae receptacles.

If you don't want to spend much $$ but want a direct Audio link between your Sat tuner and the radio, you can do what others in here did: buy a Scion head unit. I think it's plug-and-play with the 4Runner harness plugs. You can probably even sell your OEM unit for like $25 on ebay to make up some of the cost. There have been alot of posts on those in this forum particularly, so search around and you'll find some options.

Or go the FM modulator route.