What is the best way to add a sub and sub-amp while keeping the H/K HU? My dad won't let me make any real changes. I have a 2004 S.

 

Probably the best bet would be to tap a line level to RCA adaptor off your rear 6x9's and into your amp.

 

Like tampastang says... RCA output converter, you can get them at bestbuy or circuit city for about 20-30 dollars.
you connected behind the radion to the rear speakers (both of them into the adapter so you can get enouoght power) then connect the RCA cabel to your sub or amp... is very easy.
I did like a week ago to my mini and it took me like one hour.

I used this pic to make sure i got the right cables connected to the adaptor.

good luck...

 

wires running in the back right kick panel... are any of them speaker wires?

 

Nope. No speaker leads in the boot.

The rear channel wires run from the radio, over to the right side front passenger kick panel, then the left channel crosses to the other side under the seats, then each channel runs down its respect door sill, then up to the rears. In addition to the radio, you can also tap them behind the rear panels, or from one location behind the rear front kick panel.