B5 passat tdi 1997

Does anyone know what controls the 3 heater glowplugs in the cooling system?They dont energise even when its near freezing.Iwould also like to know what colour temp sender should be fitted as I suspect its the culprit even though its only just been fitted.Thanking anyone who replies in advance.

interesting first time ive heard of these?

Turn the heater to max and see if they come on. There might be somme settings that need to be changed with a vag com. Do a search on www.tdiclub.com for more info.