how can i find out what is wrong with my fiat 2.8 diselducato 14 heater plu?

i have a fiat ducato 14 1998 s reg diesel .the glow plugs are not working. i have traced the relay for the plugs & have got a feed at the relay but the switching current for the relay is not there when i switch the ingnition on.the glow plug warning light on the dash does not work.if i bridge the relay (main terminal to the glowplugs for a second or two the plugs work ok. i think there must be a water sensor switch or somthing wrong ???.please could you help.