Current status - from the mercedes wiring diagram of the 1432 it looks like the amps take line input. So, wired it up to the line out from the headunt - wired to the 4 lines (L & R) that go to the fader, routed the amp on line from the head unit to the car harness, and... it works fader and all. HOWEVER the output is pathetic. Turning the head unit to max vol gives a reasonable stationary car not running level.

This confirms what people have said that the becker amps can take speaker level inputs (also confirms that line level is not always the same).

Next is to wire to the speaker outputs of the head unit.

Paul
Update on 9/23 wired into speaker output, now things play at reasonable volume levels, mid volume on the Sony (15-16) is realy fairly loud. I know this is less desirable acousticaly as you are running the signal through the sony amp and the pre-amp outputs are more "pure" but its fine for now. No idea if using the speaker level outputs can damage the becker amps over the long term, doesn't really matter to me as if there is an issue at some point in the future I will buy a separate 4 channel amp. The nice thing for now is the original fader still works