Greetings!

Perhaps you might want to go from the factory head into the aux in of the empeg? Assuming (correctly???) that the second audio stream to the back seats comes from the head, you can have the head play to the rear and still use the empeg up front. I have rigged my factory unit with internal amp to the aux of the empeg. You can read the results here. I don't know if this is quite what you want to do (you might still need to cut the output voltage if you are going to your factory amp, and it is still not your first choice), but it should work.

Paul G.
Q# 15189
SN# 090000587 (40GB Green)