OOPS!!!
The problem is likely to be the key blocking the space where the cover goes, which is why it won't open.
OK, you probably are going to have to remove the heating controls. You will need a Torx screwdriver (T20, I think, but get a range). Slide out the ashtray, and remove the insert (I think there is a clip on the underside; release this to lift the tray). Then, on the facia just above the ashtray there are two screws to remove. Take these out, and it is possible to then pivot up the facia to let it unclip from the top of the heating controls. To get more space, you should remove the ash tray assembly; two more screws just above the ashtray (hidden by the facia). There might be two more under the ashtray; I can't remember. Remove the screws, and the ashtray assembly can come forwards. There are three wires, but if they are long enough you shouldn't need to disconnect them.
If you now look at the heating controls you will see there are four screws holding them in place. Before you remove them, see if the cover will now move (as the facia is not in the way, it might flex enough to open, or let the key drop down). If not, remove the screws and then carefully juggle the controls out. With luck you might now be able to get at the key.
If this hasn't worked you are going to have to bite the bullet and (probably) break open the cover
. You cannot remove it without opening it, as there are 4 screws holding it in place that can only be accessed with it open. Try knocking firmly (limited space, so tricky) on the underside of the casing while moving the cover to see if that works (moving the key out of the way for long enough to let the cover go under it). Failing that, have a look to see if you can see the screws through the edges of the cover (torch needed), as if you can get the driver onto them you can undo them, and get the whole radio assembly out. You might see the key, and get something to hook onto it.
Fingers crossed, and I hope you succeed.
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