If I remember correctly, the premium system amp was configured in an quasi active configuration. The fact that you have two separate wire leads tells me that there is a dedicated frequency going to both opposed to single run with a resistor in the door acting like a passive crossover. If I were you I would grab signal from the rear speakers or what I found to be better for sound quality was to grab the signal directly from the HU (pre amp) which is a balanced signal vs. high level coming from the amp. What aftermarket gear are you planning on running?

 

 

 

Call Audiocontrol and see what they recommend because most of their products will take a balanced signal and spit out something with a 9 volt preamp signal. I used to use an overdrive module and it had a switch for balanced signals.

 

The Audiocontrol Overdrive unit like most of their models, have an input selector so the question is going to be what comes out the other end and is being fed to your aftermarket amp? When you go this route you get an extra boost in voltage as well going to your amps inputs which is a win win if it works. You want to explore all "preamplifier" solutions and use high/speaker level as your last resort for sound quality reasons because there is a significant difference. Balanced is your friend