I put the same heater in my Wolverine. I still need to get my cab sealed up because it struggles when it's below 30 degrees. I may have to block off the radiator too. The heater keeps the engine running too cool.
If you routed the coolant lines the same as
@Willi then you tapped into the hot side and the cold side of the radiator loop. Your engine will still run at normal operating temperature because it is behind the thermostat controlled inner loop.
I ran my coolant loop in series with my heater core getting the hot coolant first then the radiator. This works well but in real frigid temps I blocked my radiator with cardboard and it did make a huge difference.
Sealing the cab is the real winter solution w/ heater. The cab of a p500 isn't sealed well even when you box it all in. My next project is to try to seal all the gaps in the firewall that blow cold air in at speed. Putting around it can get real hot in there. Riding at 20mph or above blows all your heater heat out via the cold air coming in.
Also,
@Willi please update on how well your set up works for you.