CamAsciak

Hi all
I recently bought a 2002 LS430 UL with the ML soundsystem. The original sub is not present, so I assume it was removed by the previous owner likely due to it breaking. I have subs lying around from previous cars so I decided to wire in one of them instead of spending money to fit a replacement in place of the factory one. I finally finished wiring up the sub earlier on running a +12v from the battery and an earth from the boot, using the remote wire from a parrot system already fitted. I didn't know which wires to splice for the audio signal but decided eventually to splice the wires connected to the rear door speakers to use as signal wires for the sub, connecting them to the high level inputs on the amp for the sub.

I finally finished wiring it all up but when I went to try it, there was nothing. I turned off the low pass filter on the sub amp and turned up the gain on the amp and the sub started playing music, but not bass so I assume the ML amp must have some sort of high pass filter built in stopping the door speakers playing anything below a determined frequency. So my question is, where do I take the audio signal from for the sub amp? I can't use the wires used to power the factory sub as the amp has built in protection features which basically make it not possible. I then thought maybe the front door speakers since the amp has separate wires for the dash tweeters but there's probably still a high pass filter for them aswell.

I figured the only other option would be splicing the 'audio in' wires feeding the amp from the head unit, but I've searched for ages for a wiring diagram for the amp and I can't find one anywhere. If anyone has a wiring diagram that shows which wires are the correct ones to try this, that would be great. At the moment, it's the only idea I can think of that could work

Thank in advance
Cameron