Having some time and no extra amps or subs lying around I decided to try hooking the stock amp and sub up to my Kenwood head unit. I connected the orange wire that runs to the stock amp to the amp remote turn on wire of the head unit. I then plugged an rca cable into the non-fading pre-out of the headunit and chopped off the other end. I spliced on some speaker wire and connected it to two mono leads going into the amp.
Voila, an occasional faint rumble when the stereo is blasted. Lots of static when I pause the music keg. I was scared to try and start the engine with it hooked up. It seems like the amp isn't getting enough signal??
I guess I'll buy an amp.

 

ok what are you trying to do... hook the stock sub amp up to the aftermarket headunit... why not just connect some rca to the sub out on your headunit.. then cut the other end off and solder it to the input signal wires on the sub amp (not sure what wires they are but i am sure if you look at the schematic it would be easy enough to find out... i have the schematic just dont have time to look at it right this second)... and i believe that plug back there is still hot and the ground is still going to be good.. just check for a turn on lead and you should be good to go...

 

That's what I did. The amp clicks when the stereo turns on so I know I have the turn on lead connected correctly. I am getting faint vibrations from the sub so I feel pretty good I did the rca to speaker wire to amp correctly. My guess is the stock head unit is sending a lot more juice to the input signal wires than your standard preout. Even with the gain turned all the way up on the head unit, all I get are faint vibrations from the sub.

 

probably because the stock subwoofer amp is extremely weak

 

what headunit are you using... did you wire the rca to the sub out.. also did you verify that the stock amp has power and ground going to it... if all else fails get a small amp and use it to power the stock sub..

 

My headunit is a Kenwood KDC-X469. It has front and rear rca pre-outs and a non-fading rca output. I hooked the rca cable to the non-fading outputs. I didn't measure the power going to the amp, but it does click when I turn on the stereo, and the sub does move when music is playing, just not enough. I am thinking of getting a Kenwood KAC-X810D amplifier. It's way overkill with 1600watts at 1 ohm stable, but I am probably going to not like the stock sub eventually too and might as well plan ahead.