Wiring stock amp to aftermarket HU

I'm in the process of installing my new stereo system, and have most of the pieces in, but until I build the woofer enclosure, I don't want to start ripping my car apart (I'm more into doing the whole job at once). But I didn't want to wait with the headunit :) So I went ahead and got the dash install kit (which, by the way, is a b**ch to find for a Lincoln Continental '00) and wiring harness, and got it all hooked up for now. My question is though - this car has the so called 'premium' sound system (that is, the Alpine sound system with 6 speakers, and a 6CD changer). In the trunk there are some actively powered speakers (with a not all to powerful amp). On the wiring harness there's a separate power and ground for that amp and a + and - for the powered speaker/sub/whatever. Since the headunit (Alpine CDA-9855) only has the normal 4 channels (Front/Rear Left/Right), I don't have anything to connect the sub channel to. Any way I can easily split that off from one of the rear channels? Hooking it to the stock speakers is just to tide me over until I get to install the new amps/speakers/sub, so if it's a totally elaborate process to get the stock amp/sub going, I'll probably pass on it. But I figured I would check here on the off chance it's pretty simple :)

Thanks much for the info folks!

Since you are connecting to an amp, you can use the RCA outputs from your H.U. Metra makes some harnesses for premium sound systems with RCA and regular speaker connections, so that may be an option. The easiest thing to do would be to splice a spare set of RCA cables into the factory wiring to the amplifier. If you've never cut and stripped RCAs, they basically consist of the visible insulation, an outer metal shield under that(which will go to your negative lead), a layer of plastic, and the innermost strand of wire (which goes to the positive lead). You can use a Y-adapter, or just splice both channels' outputs together so you have sound from them both. C Renner's Auto Electronix

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i would suggest  the same thing. i have done a few installs where .i have done that for a factor sub. but as long as that factory amp has power .its as simple as getting signal to it.1974 ford pinto 4 15" punch Z power punch bd 1001 pioneer DEH-6700

Thanks for the info guys - I just hooked up that part of the harness to two rca connectors that are spliced together, and it all works like a charm. This'll be fine until I get the rest of the stereo installed.

Thanks!