Normally the control boxes rarely die. It´s mostly dirty fuel that causes trouble. There are a few standard failures that are easily sorted out...
Webasto aluminium Control box My problem was intermittent, after the ignition cycle, I had to bang on the unit in order to get the fuel ignited. I decide to change the 1995 Aluminium control box with a new epoxy. The Webasto heater is working like a charm. The Webasto technician confirmed that the old control box model, had problem with cold solderings and that the new generation of control boxes are filled with epoxy to prevent this problem. Thank you for your comment. Kind regards, Normand
Well, the old non-electronic version is with a number of relais type switches inside and with salty humidity they can start producing contact problems. But that´s very rare. to me it sounds like your ignition relais has trouble or does not get proper signal from the flame control eye. You can check that with a voltmeter while starting the heater unit while having it open. Be sure you discommect the cable from the electric fuel valve at the injection nossle before. Cover the flame control eye or the ignition will be switched of and be careful with the high voltage side. You might also check the plugs under the ignition coil to see if there´s oxidation. There are a few cheap tricks to cheat the control box for diagnosis purposes, but you have to be careful with the high voltage and fuel spray.
Repaired a friends. Removed case found cold solder joints. Re-soldered, reconnected he was back making heat.