Yes, I reused the stock center speakers and stock center channel amp when I put an AVIC in my wife's car.
The connections are on the 8 pin connector, with the sub woofer connections. Pin 8 (yellow wire) is the amplifier enable pin. If you drive this from an aftermarket radio, you need to put a 1K ohm resistor in line to prevent the amp from popping at turn on. Pin 5 (white wire) is the positive center channel input, and pin 6 (gray wire) is the negative center channel input. Most aftermarket systems don't have differential outputs, but this is okay. The factory amp seems to work just fine if you connect the negative input to audio ground and the positive input the the audio out. If your aftermarket system doesn't have a center channel output (most don't) you can at least rig a mono output yourself. Take the left and right outputs and connect a 100 or so ohm resistor to each. Now connect the remaining end of each of the two resistor to the positive audio input to the amp.
