coolant heater?

Iv'e got a 1996 GMC Serra diesel. Other than the usual problems with the cold weather, like using fuel additives, changing filters, and cursing the cold; I don't think my coolant heater is working this year. Is there a fuse or relay for this? I've looked but cannot locate one. I've tried a heavier extension cord but no luck. Thanks

If its been in there since 96 I'll bet its burnt out. The one in my 1990 lasted about 6 or 7 years.