The Monsoon speakers in my VW were held in place with solid plastic things
instead of screwes, and they had to be drilled out to remove the speakers. The
dealers might have a tool for this, but drilling them out was the quickest way
to get the speakers out. A word of advice though: Monsoon systems have
specially matched components and a weird amping/crossover system similar to
what Bose uses. Many of Monsoon's systems, especially the ones used in GM
products, have the rear speakers wired in to act like tiny subs, and switching
speakers to a non-Monsoon brand may mess up the sound. You may want to consult
a Buick dealer about Monsoon brand replacement speakers.
Steve Grauman