On 306's and (most!) 106's, probably Saxo's too, inside the radio arpeture there's two nuts holding the top of the heater / vent panel in place.
However on mine, these are not present. There's some kind of (not sure how best to describe it) a open ended figure of eight spring clip. I had a mirror and torch in to see exactly but I could not move them with various pliers. They don't appear to be snap fit to remove either as I'd already applied a large amount of force to attempt to move the panel.
Any thoughts please ? , Generally I am finding some very odd (but not in a bad way) differences between this one and say a V/W plate model.
I built it back up but when I have a go again in a few days I'll try to take a picture if needed. I think I'm missing something obvious really. From what I can see in theory squeezing the O part should move the tangs out (on the outside) and compress the other end allowing removal but tis difficult to see and even more fiddly to work through the tiny letterbox