For the 240, my wiring diagram shows the R stop to be Yellow and L stop Yellow/Grey. The backup lights are on a black wire.

Since all these problems seem to have happened together, I'd suggest checking the connections back at the rear. Each taillight combination has a flexible printed circuit board. The sockets fasten to these "boards" and the power comes in through connectors which go to the copper circuits on the boards. This copper can corrode and make poor connections. Also I've had the wire connectors make poor contact at the boards. You should start where the wires come in from the front of the car and clean and tighten that connector first. If it was loose, it could affect both stop lights and the backup lights. Check the voltage back at these wires at the connector to be sure there is no break forward of the connector.