Hi, we have just purchased a 2003 Alphard camper. We are still trying to figure everything out but we are stuck on how to use the rear heating without the key being in the ignition. Could anyone help please? Is it possible to do or not?

I think the engine would have to be running to supply heat; the only other source of energy for the heater system would be the battery but that would quickly go flat, so yes, in common with most if not all cars you will need ignition and engine on for heating. You can install kits designed for campervans but they’re expensive, cheaper to put on a jumper and a hat.

Thanks @wjvh I will have to look for an alternative option. 😩

This sort of thing; will need to be professionally installed and certified; will draw petrol from existing fuel tank. Good but expensive .

https://www.eberspacher.com/products/fuel-operated-heaters/air-heating

Diesel, I think, not petrol, but if your vehicle is petrol a separate tank can be fitted. 

Yes, oops, thought those were petrol. Try this: 

https://www.butlertechnik.com/webasto-air-top-heater-2000-stc-petrol-gasoline-kit-12v-with-rotary-rheostat-4114674a-p39/s1782?utm_source=google&utm_medium=cpc&utm_term=webasto-air-top-heater-2000-stc-petrol-gasoline-kit-12v-with-rotary-rh-41sk464&utm_campaign=product%2Blisting%2Bads&cid=GBP&gclid=CjwKCAjw9aiIBhA1EiwAJ_GTSjuBb_7oheotnkvlmcLGf3if8_WS1gffSOoHqERUgJyEbl7D7AS8qhoCvnAQAvD_BwE