Webasto makes an air heater, and you can find cheap knock offs from China through
Alibaba. These things will roast you right out of small 4Runner. But at least you won’t
have condensation inside.
Are you wanting an engine diesel coolant heater? Or an air heater? They can be expensive. Look
up Webasto or Espar
http://www.espar.ca/site/coolant-heaters/
https://www.webasto-group.com/en/original-equipment/thermo-systems/fuel-operated-heater/