After a struggle have suceeded in removing back seat, The Haynes manual says it is simply a matter of levering off a circular cover but I have found a triangular plastic cover rivetted into place over this, what is this for? Is it alright to drill out the rivets on this? Thanks for any advice you can give me, all the wiring on this car seems so shaky that despite being a retired electrician I am nervous oabout touching anything!