Several things come to mine.
Number one is DIESEL ENGINES HAVE LESS WASTED HEAT because they are more effecient

Big lazy gasoline V8 engines put out more waste heat that make the cabin nice and cozy
Number two is even with the viscious heater there's not a lot of heat unless the engine is at operating temperature. They can idel all day and not warm up and cool off waiting for a stop light.
What was your temperature guage reading in warm/hot weather. If it was lower then one tick left of center your thermostat is bad. When mine was failing temperatures had to be above 90 degrees, on a interstate with the AC on before it would get to operating temperature

It's easy enough to check if they plugged the viscous heater in but remember it has to be below a certain temperature before it operates and when it does the heater is just barley warm.
Lastly I think at least one person put the hoses on wrong when replacing their thermostat and had no heat
