Well firstly youve got to add a heater tap to the head and run a hose from it to inside the cabin. Then you've also got to change your bottom radiator hose so it has a take off hose for the heater. These hoses both run inside the cabin through the holes in the passenger side bulkhead.
The heater tap has a wire which will need to be run into the dash somewhere.
The only wiring needed for the heater itself is a switch to turn the fan inside on and off.
Ive only ever dealt with clubman heaters and they are awfully large, so nowadays I just run a small fleece blanket for the passanger
