Take off one of the wires to a heater plug. Put it to a single contact bulb, (earth the case) and when you switch on (you need an assistant obviously) Time the how long the light stays on. If it goes out after a very brief time it is the glow plug timer that is faulty. On my F350 that was my fault, and I wired the heater relay to a switch in the cab so that I (can) time the actual heating period. just my 1penneth. good luck, keep us informed, though hopefully some others with this actual vehicle fault might step in with some advice.
