You should be able to pick up a used stock heater cheap, if that is suitable. It mounts in the engine bay. It seems strange that your truck is without one.
If you want custom, see the link to my install above and some options. Is your truck right or left hand drive? That impacts options quite a bit.
Pissing people off on the "net" since 1983.
Land Rover. Turning owners into mechanics since 1948.