I have recently experianced same problem where glow plug light comes on after engine is started. Found 2 plugs showing high resistance. Replaced all 5 and to my suprice light came back on after engine started and stayed on. Pulled the relay out and found out that one of the plugs fuses is blowed. OK, repaired the fues but relay wont work. It turnes out that one of the plugs is reading 0.4 ohms which should be below the tolerance. The relay probably switchis itself off detecting the overload. Would somebody know from the pinout of the relay what plug is connected to what pin?