head unit to low level input on stock amp

Initially I thought I could just drop a new head unit in my 1995 XJ12 with not problems, justy use a wiring harness and mounting braket. Wrong. Being an XJ12 it comes with the Harmon Kardon sound system. I knew it had an amp, what I didn't know is that it takes low level inputs from the hed unit in a plug similar to an AT keyboard type plug (circular plug with 6 pins). It is wired so that there is a positive input from each channel (right front, left front, right rear, left rear), and a common ground for all. Also in the FSM wiring diagram is states 0-30MV active. I assume that it takes 0-30 mvs? I have a head unit with pre amp out puts for all channels. I intend to wire teh head unit to the factory amp. I know it isn't exactly a perfect way to do it now, but at this moment all I want to replace is the head unit. So, what I would like to know is what do I do with the common ground? Do I merge all the negative signals from each channel comming off the head unit? Take only one of the negative signals off the head unit and put it to the common ground on the amp? Which is the negative comming off the low level RCA outputs on the head unit, the inner pin or the outer ring(IIRC it is the outr ring)? If this can be done, are there any other road blocks I would likely hit, like the voltage on the low levels not being sufficent or too much for the amp? -Chris

Decks dont run common grounds nowadays. Pretty plain and simple, dont do it. You can cause some pretty bad damage to the head unit on this one. You can check and see what impedance the speakers are, you might be able to hook to them directly but if you cant then you are going to have to either replace all the speakers in the vehicle or put in aftermarket amps. Sorry I couldnt be more of a positive help.double-secret reverse-osmosis speaker-cone-induced high-level interference distortion, Its a killer

How do I check the impedance of the stock speakers? Just use a multimeter set to ohms? I guess I will, for now atleast, run leads from the head unit to where the stock amp is and and wire them to the existing speakers wires coming off the amp. However, how do I deal with the tweeters and teh mid ranges? I was htinking throwing a resistor in line for the tweeters.. but what value? Also, the stock HK system has a 6"x9" sub woofer... what the hell do I do with that if I cut out the factory amp? -Chris