That would only work if I was dealing with speaker-level outputs from the Empeg. But unfortunately my problem is the opposite (and one which an amp and crossover would solve). The marked speaker-level outs on the main harness do seem to require speaker levels but only appear to feed the tweeters. If I connect the 4v line-level outputs of the Empeg to them, I only hear very low volumes from the tweeters (as would be expected if they were wired direct). The secondary wiring harness adapter is wired with only 4 wires (out of a possible 8) and marked with amp enable and ground and sub enable and ground (no subs in the car). However the harness in the car has all 8 pins connected. My guess is that it is for the 4 channel low-pass line-level signal to the two amps driving the 4 mids. And finally there is an extra wire coming into the main harness that is not connected on the adapter. I'm guessing that one is the real amp turn-on wire. One other data point is that the '00 and earlier Mustangs w/Mach 460 had an amp behind the dash under the head unit. My 2001 does not. If I can figure out exactly how everything is wired, I do have another option. If I don't mind giving up the front/rear fader, I could conceivably use the Empeg's EQ and create a low-pass setup for the rears and send those to the existing amp/mids. And then create a high-pass setup for the fronts to be routed through a new amp to the tweeters. That would only require one new amp (hopefully installable behind the dash) and would use all of the existing components.