Is this plausible?
Older diesel engine vehicles are more reliable under EMP conditions because they lack the computerized fuel management and ignition systems of modern gasoline engine vehicles. However, that does not mean that an ungrounded diesel vehicle would still start! It would run, but getting current to the glow plugs might well mean the use of a small, easily made jumper cable. Note on the diagram below the controller, sensing resistor, dropping resistor, thermo switch, relays, etc, all between the battery and the glow plugs. All those susceptible components can be bypassed by using a jumper cable from the positive terminal on the battery to the glow plug buss.
A jumper cable between the positive terminal on the battery and the glow plug buss (see arrow) will heat the glow plugs when the engine is cold. The hood is opened, the jumper cable attached, run back inside the vehicle and start the engine, then run back and yank off the jumper cable. Bingo, your diesel engine will start and run!
You will want to be pretty quick when using the jumper cable. Each glow plug draws about 12 amps, so a 4 cylinder engine is drawing almost 50 amps through that 10 gauge jumper wire. Once the engine is running, you will want to pull off the jumper cable as fast as you can. As long as the engine is warm, it will start without the use of the jumper cable and glow plugs. The radio and some other electrical devices would not work, but the vehicle would run!