Did you buy it new? Have you ever changed the thermostat? Has the system ever been back flushed? A 195 thermostat will help a heater put out more heat than a 180 thermostat will. If the stat has been changed and the heater still sucks, I would look at the blend doors in the heater unit and be sure all the doors are opening and closing like they are suppose to.

I've also heard the the radiators are so big on the diesels that the coolant never can stay at the stat temp. On those, you have to restrict the amount of air passing through the rad in the winter. But then, that falls into the troubleshooting area I mentioned in the earlier post, doesn't it? Gene