I have used both 2 channel and 4 channel input settings on the h800. When I used the 4 channel input settings I was able to retain fader control on the factory radio.
I have used the h800 into different systems. One of which is 2 way active front stage with a sub and optional rear speakers. Another is 3 Way active with rears and a sub but the midbass are located in the rear and ran mono on channel 7. You can see both installs with links in my signature. The Scion tC has the second system describe and ran and ms8 previously.
Rich V.