tarvey

I have a problem on my 2000 Galaxy.Normally it is a good starter but sometimes when i switch on,the glow plugs light briefly and go out and it will turn over but won't fire.If i open the bonnet there is an 2 electrical boxes ( i assume they are relays)if I tap them the engine will start. Question Is one is the relay for the glow plugs and which one is the correct one.