I'm hoping it's your lucky day. I haven't been reading the list a lot lately, but I knew right away what the problem is with your Vanagon Diesel. There is only one electrical draw on an old VW diesel with tens of amps: the glow plug relay. Yours is fused permanently on.

Why isn't the glow plug light on then? Because the light circuit is only one part of the relay. It is independent of the relay supplying power to the glow plugs. The relay is designed to supply power for several seconds after the light goes out to assist during the starting process. So while the light may work normally (part of the temperature sensing circuit), the part of the relay that switches the power to the plugs has "welded" itself together. Probe the glow plug bus bar with a simple 12V test light on a warm or non-running engine to confirm my suspicions. There should be no voltage.

I had a 1980 Dasher diesel wagon. (Loved it BTW). In any event, my lights were dim when the car was running, and the analog clock would actually stop clicking a few minutes after the car was shut off! Completely dead battery of course. Attaching the cable to a charged batttery would cause a significant spark due to the amount of draw. End result was the glow plug relay. There is nothing else on the car that can pull that kind of juice.

I've found the glow plug relays on just about all of the old VW diesels I've owned to function inconsistently from one another. Some run the light for far longer than they should, another kept the plugs hot for almost 2 minutes after startup, etc.

The temporary solution is to insert the relay only when you need to make a cold start and then remove it. I'll bet one 1986 Jetta turbodiesel without a drivetrain the relay is the problem.