Had no heat on the driver's side of my 1997 S500 coupe. I was able to get a rebuilt duo-valve locally, swapped it out and now have heat.
The duo-valve solenoids de-energize to provide heat, and energize to reduce the heat. I tested the old valve by putting 12V to each of the solenoids separately and they didn't sound the same - one had a nice resounding click and the other was quite muffled. To do this test, you unplug the connector between the solenoids exposing the three conductors. The middle on is the ground for both, so you provide 12V to one of the conductors with the ground on the middle and listen.
Over time, the pistons that are moved by these solenoids gather corrosion products and tend to stick in place. This can result in heat or no heat depending on what position they stick in.