I have a 2001 Tribute with the 7-speaker stereo and factory subwoofer in the load area. When I got the car, the factory headunit had been replaced by an aftermarket but it was well past its best. The turn on lead to the rear subwoofer hadn’t been connected so the subwoofer was idle.

I replaced that headunit with a Kenwood and connected the factory amp turn on to the Kenwood power control lead, the sub turned on and played, all seemed good.

I’ve now found that the factory amp lead doesn’t just get 12V to turn on, it gets a 5V signal to turn on, then the voltage rises with selected volume on the factory headunit. The amp interprets this signal to turn down the amp power as volume rises to prevent clipping - but it means the amp with 12V is only putting out low power all the time.

To fix this I recalled some school physics and made a simple voltage divider using 2x 1000ohm resistors and a short section of chocolate block connector as pictured. The outer connections take the ground and headunit turn on 12V, the centre tapping is connected to the factory subwoofer amp turn on lead and gets approx 6V all the time…

…noticeable improvement in subwoofer power :-)

Here is the connector with the resistors in place, simply wire the other side:

One end - Headunit amp power control 12V signal
Other end - Ground (either harness tap or chassis)
Middle - factory amp turn on lead (blue/orange wire from vehicle wiring)