Junkyards have SSE and SSEi Bonnevilles all over, but I suggest going with an aftermarket speaker set for even better sound. The problem with that is you have to take off both of your door panels for that switch. You could at least go to a junkyard to practice taking off door panels, though.
The 96-99 models are the ones with the newer style front grille, headlights, and tail lights. Actually, you could just go by whichever Bonneville has the BOSE logo on the front, under the radio. That works just as well as identifying by the exterior. There are either two 10mm bolts or 2 screws in each door panel. One is under the plastic part at the top of the door handle, and the other is under the light fixture under the door handle. It should be either a T10 or a T15 that holds the charcoal piece surrounding the door handle, and everything else is just plastic pins.
94 STEi and 99 SLO