The opening is found when you actually remove the glovebox lid completely, ie the 2 pins and drop the lid, behind it there is a wee flap held closed with a screw it comes off but i dont see any reason for this panel. The glovebox on this car is also a bit trickier to remove than mine, the screw in behind the glovebox lid that is supposed to go into a metal bracket has its head orientated so you need to get at it from the other side (ie the side you cant get at) and there is also a screw down the bottom near the middle of the dash holding it in which i didnt know about and broke
Fuel cut off switch, were peugeot afraid of these pre multiplex 406s with their at the time new HDi engines doing diesel runaway on them
As for the rust situation, the boot floor is rusty because the boot didnt shut (it had an accident involving a skip and an icy road), its had water sitting in it which has corroded it, the sills are rotten most likely because the car was jacked up and the drainage holes blocked, but its not really a rust bucket, here is the rusty sills:

(hard to see but basically the sills have been half repaired and stone chip painted over but they didnt get through the MOT
Under the carpet in the rear passenger footwell there is a bit of rust that wouldnt take long before it is a hole:
The engine seems to run good, the battery is knackered, and there is a possibility the coolant got a bit hot at some point:
Nonetheless the plan is to continue taking off the parts, shout if there is anything that you need.