My driver side front door switches have stopped working. Here is what I know:
1. speaker still works
2. courtesy lights work
3. I have power at the connector between the door and the door panel.
4. I swapped in a DDM from a junkyard and nothing changed. I don't know for certain if this DDM is good though.

What else can I look for? Bad ground somewhere? I checked under the rear seat and there is no water (I've been out in New Mexico for almost a year now). I have a Helms manual but I'm not sure where else to trace the power to.

You say you have power at the connector is this the one before the DDM or after?

Good question - This is the connector from the door harness to the door trim panel. I haven't traced the schematic to determine if it is pre/post DDM.

I did some more reading and realized that the driver door switches (locks, windows, mirror) don't directly connect power to their respective motors (they don't drive relays or transistors). Everything is digital. There is a microcontroller in the switch assembly which interprets these voltages and sends commands through a serial bus to the driver door module. Since I can lock/unlock my car with the remote, and the driver door locks and unlocks, I believe my DDM is good. I have power and ground going into the door panel at the C503 connector. I believe the switch assembly is bad, and it is not sending serial commands back to the DDM, or the wire harness in the door panel (connecting C503 to the switches) is damaged. I will check the harness, and if I can't find a problem, then I will order a new switch assembly. I think this diagnosis explains why only the driver door doesn't work. The speaker feed works, the courtesy lighting works, etc...

Update:
It was indeed the switch assembly... dropped in a new one and suddenly my switches work. Hopefully this will help someone in the future when they are trying to diagnose a DDM.

Thanks for the useful information, same thing happening here, they light up but don't function. I figured the switch panel was the likely culprit and I actually took it out earlier to take a look. I just bought my SSEi yesterday and have many other projects to complete, this is minor but very useful to know. I am going to see if I can troubleshoot the board itself. I will update if I fix the circuit board.

Yea them switches will do that they hate the rain