Last time a member tried getting that plug the dealership only sold it in an entire package... something called a 'theft recovery' package for harness's that had been cut. But if you can get your hands on them then that would be awesome.
The amp's that will work for are is any amp after a 2005. The two amps that have the voice out I have here are:
Seqouia: 86280-0C050
Tundra: 86280-0C070
My current 4Runner amp which is an 06 amp I don't remember (sorry!) It's in the car and I'd have to remove my panel, etc. I plan to remove that panel here in a few weeks so I'll get it for you when I rip it open. But both those amps above do not have a center channel so it would be ideal for your current setup.
Oh, btw, I've seen some amps on ebay recently from 4runner's that have this amp 86280-0W150. This DOES NOT have the voice outs that you need.
Sorry I couldn't be of more help.
j