The pedal switch reports brake action to the front Vansco multiplexing module (VMM) behind the plastic panel in front of the passenger seat. This front VMM reports the pedal action to the rear VMM (in battery comparment) over a twisted pair wire cable along with a host of other signals running the chassis. The rear VMM tells the brake lights to light; this brake light signal is reported on the rear VMM panel with output LED #20. You should be able to verify this by having somebody press the brake pedal, and watch output LED20 light up on the rear VMM.
Because of the electronic wizardry in the VMM boxes, wiring directly off those pins isn't practical without a greater level of knowledge than I have, and it isn't necessary if you use standard processes to wire up the trailer.
IWM I'd tap into the brake light wire at the 6-pin plug with a relay (double tap @ the "back" or forward-facing side of the plug), using the brake light on/off status for the relay's signal leg. I'd take 12V+ from somewhere practical (take a look at the fuse block on the back wall of the battery compartment to see if there is room, otherwise add a wire w/ring terminal to the chassis positive stud in batt compartment & use an in-line fuse) to run the trailer brake circuit, and run it thru the load side of the new relay to the trailer. I believe you can use the Auxiliary pin on the 6-pin plug to carry the trailer brake power from this new relay. This adds only the amount of load to run the new relay to the VMM #20 output circuit which should be easily managed.
Remember that pins on the 6-pin plug are energized thru the VMM system, so the status may vary from the coach sitting w/key-off to a key-on condition (which sends additional instructions thru the VMM system) when running. I.e. do all your analysis w/the ignition key on so the VMM signals will be in a run mode.
Baja-tested '08 2-slide 36'
Alpine: The Ultimate DIY'er Project