air con - what exactly is going on...when on?

[Podge]

It might be linked to the air quality sensor under the plenum chamber (the grille at the bottom of the windscreen) it's this that senses poor quality air and sends the signal to go to recirc. Perhaps the signal from selecting reverse goes via this sensor too. It's in a damp place and the wiring or plug can get corroded. It usually has a fault code associated with it.

 

[Alster]

You’re not over thinking this and it’s your rear door seals??

 

[Spaghetti]

So on the button record works fine and I can clearly hear audible change in the cabin when the presumably flap closes the intake from outside. However in reverse there is only a minor change and I instantly get exhaust fumes in the cabin. Where previous it was a notable audible change with a bit of a thud from the flap as it shut quite quickly. The reverse light work fine too, as I’ve upgraded them to LED in order to see where I am reversing into when looking into a rear view mirrors.

Worth checking your EGR pipe in case that’s cracked too, as that can allow fumes in.