high pass input problems with amp

I'm setting up my subwoofer in a Honda del Sol using high pass inputs for the amp.  I've done it the exact same way with 2 of my previous cars with the exact same equipment.  However, this time, when I turn the key in the ignition, the radio turns on, but I get no sound from the speakers and no power to the amp.  If I go to the trunk and unplug the high input jack from the amp, the speakers will kick in.  If I then replug the input jack into the amp, the amp gets power, the subwoofer kicks in,  and everything will be working fine... until I turn off the car.  I will then have to do all of this again to get my stereo working.  Anyone ever heard of anything like this?  Any suggestions?  By the way, I'm just tapping into the rear speakers for input.  Is there anything that is sometimes tricky about this?

Thanks.

Thanks for the ideas. First things first, I'm gonna rip the car apart again to be certain that the polarities are all correct before saying that they are. I checked out a honda page that described the colors for each speaker wire. I'm pretty sure that I'm hooked up the way I should be, but I'll want to double check. Maybe I have some wires touching, because I did a fairly makeshift job on my connections.

Thanks.