Glow plugs light will not work?

Hi. I have a citroen Dispatch, 1.9 diesel, 2004. The glow plug light does not come on and neither are the glow plugs energised. I have loacted the glow plug relay and found a broken earth wire. I have repaired this and all was fine for 1 day. I have now check the glow plug relay and wiring and have got the following.:-When iginition turned on there is power to the glow plug relay. The earth wire is intact and connected to earth. The 2 control wires that feed back to ECU are in good condition and I have continuity at both ends on both wires. The main feed wire to the relay has constant 12 V but the relay does not energise so no power getting to glow plugs, When I manually feed 12v to this wire on relay, then glow plugs work and van starts. I have replaced the relay and I get same results, relay works fine on another vehicle. The temperature gauge works OK. Everything is as it should be. Funny thing is though, I have latest diagnostics equipment and cannot comunicate with vehicle in either Citroen mode or OBD mode. Anyone any ideas......