LR has just launched a new (to LR, but I believe some other cars have them) style Positive Temperature Coefficient heater (PTC! and don't ask because i don't know, but you can Google it!) ceramic heater that is designed to fit into a car and run off 12v circuits. It gives instant heat and lots and lots of it in my Evoque, it doesn't need the engine to warm up first as it is electric not coolant powered and gives heat from first turn of the key. Might be possible to make one fit a Defender, but also I suspect it might be out of budget. Also there are only 100 Evoques registered to private buyers in the UK 6 weeks after launch.............. not too many have reached the scrappy's yet