You really need to address the water leak situation first,a damp/wet interior will
always steam up.
Worst still,in really cold weather you can get ice on the inside! Remember they have a lot of exposed cold surfaces for water to condense on.
Stop the moisture ingress and have a toasty truck!
Open a vent glass to increase 'flow through' to stop the humidity levels building up,and even the original heater should be able to do it's thang!
Jock
