I do have another thread (with too many words!) where the traction control relay was causing all sorts of fun stuff to happen. (Other thread) The repaired / refurbished RCM (with trac ctrl relay replaced) is now in the car. (Thanks to tighed1)

It will not shift from park unless I override it with a tool through the slot. The shift indicator on the display does show P for park. If I override, all shifter positions are displayed correctly. Once in Drive, I cannot put it back into reverse without shutting the car off and trying again with override - Or, removing the tool from the slot - that seems to let it shift back into reverse or park.

Brake pedal switch is new, but I did not have it installed when I replaced the RCM and started the car. Generic code scanner was throwing a P0600 generic CAN bus comm problem? I don't have a merc scanner. Crazy question, how many sets of wires should be going to the brake pedal switch? I only found one plug into the switch, but there are two receptacles on the switch.

Is there an brake interlock solenoid under the hood, or at the shifter, transmission control module in passenger footwell? Had the center console apart enough to see that the shifter assembly is dry - I don't think water is an issue, never had a spill there. This problem happened roughly the same time as the RCM needing repair. I was stuck with no crank no start, and once it did start, couldn't shift into drive. Fussy little booger.

Ideas??