where is the heater plug relay on a N reg. Sprinter 208D?

Where is the glow plug relay found on a 1996 Mercedes Sprinter 208D.It has been behaving strangely for a while,the indicator light stopped lighting when the ignition was turned on,but the heater plugs still worked,but the light would then come on once the engine was running for about 30 seconds,but now it will not start without a bump start,someone has also suggested a crank sensor may be the fault?