A common technique with the Eberspacher Airtronics was to mount under the drivers seat, but EXTERNALLY.I would mount it inside maybe under a seat or the bottom of a wardrobe, they suck the air in at the back, heat it and blow it out the front, mine blew straight into the living area but some use the ducting.
I would mount it inside like this because it’s going to be recirculating the air, warming up warm air so to speak
Mounting it outside it will be sucking in cold to freezing air which will make it work really hard.
the noise isn’t that bad especially if mounted in a cupboard.
The hot air outlet would be routed in the space between the front seats. The fresh air intake would not be left to just use air from outside but routed to the driver door step, so nicely out the way but recycling internal air for efficiency.