Stephen, the recommended interval to check and top up the diff is 10,000 Klm and to change the diff
oil at 20,000 Klm.
Thats a longggg time in the life of a cobra.
Can't remember how much the heater cost..but it comes in a kit with wiring harness, switches etc. The model I have fits neatly under the dash and has two outlets for the demisters and a vacuum operated trap door for interior air.
The motor is 3 speed and I swear that it'd blow a hole in your windscreen on full.
Give TCR a ring, I believe they carry them in stock....very popular with the hot rodders.
Cheers