Theres also a tool you can get thats an extendable boom magnet that should stick to the keyring and atleast show you where it is, you snake it in there then fish it around until you hear the keys move or stick to the magnet, then slowly retract it back out. Thats what I have to do most of the time, works best with nuts or bolts or screws that fall behind something too narrow to get your hand through.
You can get it @ AutoZone for $10> I believe.