Glow Plug relay

I tried posting this a bit earlier but must have done something wrong so will have to do it again !!

I've a 1.9Tdi which has been a bit poor in starting over the summer. I changed the glow plugs but didn't notice any particular improvement and now that the cold has arrived it was quite clear that the problem lies elsewhere.

I think the problem is the relay as I now realise that I cannot hear it operating and a session with a DVM this afternoon showed no voltage at the plugs - the idiot light does show for about 3 seconds.

But I don't know where the relay is and Haynes doesn't help (does it help with anything?). Can someone point me in the right direction please - and also at the same time, is there a good source of the wiring diagrams anywhere on the net ?

Thanks for any assistance

Rob