I've just replaced the head unit in my 93 RRC that has the high spec subwoofer and speaker amp audio setup.
The normal stereo feeds a low level input to the amps, I cut the connector plug off and soldered on RCA connectors, my head unit has connectors for front/rear/sub RCA outputs so this was easiest option for me.
My headunit now drives the factory sub and speaker amps, think there are some photos in my members vehicle thread.