It can be done but it would require some knowledge in electronics and good detective work.
Having the schematics would make things easer, too. But if you can't get them, this is how I would do it:
Open the unit.
Follow the speakers outputs from the connector or wires to a big chip (the audio amp) inside the HU.
Write down everything that's written on that chip and google it to find any info on it, like a schematic from an application note or something. That will help you locate the chip inputs and a good place to tap a 1K-2K 1/4w resistor per input.
Don't touch or remove anything else from the board.
Look at the image below to have a better idea.
The Buffer Amp is the audio amp chip, pins 2 and 6 are the audio inputs.
This might not work if you remove the HU's front control board, that's because the microcontroller in it turns the audio amp chip on/off. It all depends on the HU you have.