You're not going to get 2.5" of clearance in the wing wall. The top layer (that you see) is 3/4" thick. behind that is structure that is filled in, but there is a steel rib from the chassis that runs at the same shape as the chassis door...if you route any deeper than that top layer, you can get into the steel rib that extends along the cab wall.
I would recommend mounting the speakers on the wing wall without removing them. Plus the wing walls are screwed & glued so it would be a mess. You could cut the vinyl and use a plunger router bit to clear some space...but I would go no deeper than about 5/8".
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