my buddy has some subs that were in his basement and they had the grill covers build right onto the sub they wrent removable he had to go out and buy some new ones when he got his new subs. you may just have to remove the whole combo as one and just buy new grils.
if it is anyhting like my freinds altima 97 then in the trunk under where the speakers are are some little clips holding the cover down hit those up with a hammer and stick a screw driver between the seat and the rear deck cover, just keep prying and it will come up, then youll have access to the speakers