check your battery connectiona and battery again,,,do your headlights also dim or flicker...
there will be one heavy red cable that is your battery voltage going into the box to power the glow plugs when activated, there wil be a plug with two white heavy wires which feed 2x glowplugs each when activated these will be internally connected to the red wire when internal relay operates!! and there will be a plug with about 9 wires if i remember correctly which are the ignition feed to the box plus various inputs coming from the microswitches found on the injection pump and also coolant temp sensor etc. there are different variations of the what is called the pre/post heater module,,, mines was linked to the exhaust gas recirculation system and mines also isnt a
first of all id try to jump start your car to know for sure the vehicle is getting enough power. then work from there to see if the box is not powering off and draining the battery,, also check the charge output from alternator whilst car running put a voltmeter accross battery 13.3volts should be minimum.
on checking glow plug power remember as apre and post heater this unit should have a cycle of turning on and off for about 3 mins before engine is warmed up and isnt required anymore. this is because of this particular engine is quite "clattery" and smokey when stone cold and this box helps it satisfy emmissions
good luck and merry christmas