Heater tap = yes. All air that is drawn through the fan goes through the heater core whether or not the heater has been selected on or not. The heater tap position determins how much hot water is flowing through the heater core and therefore the temp of the air exiting the heater box. Same deal for aircon except it also has to pass through the evaporator.
An option that I have seen is a pair of 5/8 inch tubes that route the water against the block below the exhaust and enter the heater box upwards against the firewall via rubber heater hose. The heater tap was a cable operated type attached to the firewall rigged to operate in correct sense. A lot of stuffing around but the guy who owned it thought it was worth it.
You can buy just about everything else, so why wouldn't you be able to rig a 12V heater together? There'd have to be something out there you could adapt if you wanted to go that far. As long as you're careful with the amount of heat they can generate near your plastic heaterbox...
Regards,
Dave






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