It can easily be added. There is a small blown air unit designed for motor homes that fits under one of the seats very easily. I had one in a Dandy Delta and some friends have it in their Pathfinder. For the life of me I can't remember the name of it though.I have seen them going on eBay. New, they are over £400. I'm sure someone else will join in with more details.
thats what i had in mind ,i see many people use fan heaters/halogen heaters but personally dont like the idea of that--{whats that melting smell?}type of thing
The beauty of the blown air one is that you don't need to be on a hook up as they work on gas and just need a healthy battery to work the blowing part. No worries about knocking over. It vents outside too so all you get is warm air and lots of it. When I find the booklet that came with it I'll come on with the name and address for you.