Disconnect the battery for half an hour before touching airbag/wheel. To remove the centre airbag housing their are two allen key screws that are secured from the bottom side (floor) let these go then very gently remove the assembly. The rest is pretty self explanatory once you see it, if you do disconnect the airbag the utmost of care must be taken when handling it. Ensure it is electrically reconnected when putting it all back together, other wise you will get a warning light once you start the car. Mate this advice is given on the basis I presume you are talking about the centre steering hornpad area, when you are referring to cap, am i right?
Cheers Mat.
