You said it had power undercarriage wash, and when the rinse was at the front, your RPMs surged and the tranny light came on...

it is clear that the rear seats have nothing to do with this, nor the rear PDC...

you should check for loose connectors in the ABS module, in the speed sensors around the front wheel hubs - the water jets may have dislodged the wiring, and when the module is not receceiving the proper signal/information, it will cripple the system, in order to protect it...

lift the car, and visually trace the trajectory of the water jets entering undercarriage and see if there are any wires in the path - the undercarriage should be protected by a shield pan on the bottom, but i have seen many bimmers missing that cover in action...