Are you getting full head on legs up front? I've had an issue with my 300SDL, where the driver's side diverter flap did not close and I had no heat on the driver's side front OR rear.

In case the designs are similar, and I don't know that they are, try this quick check. Feel under driver and passenger footwells for heated air flow. If they are getting heat, I'm guessing the rear should be getting trickle heat, and if they are not, I'm guessing a diverter door problem.

It might be a place to start...

Best Regards,
Jim

P.S. I assume that there is nothing under the driver or passenger front seat, like stuffed animals, etc. that is blocking the air flow....