The other possibility would be to install a speaker on top of the dash,
firing up into the windshield.

Funny thing, that was my first thought as the Suburu actually has a storage
compartment in the center/top of the dash. There is a removable cover (this
is currently where my 7" LCD resides). I thought this would be perfect as
the cover could be replaced when the center channel is not in use. But my
wife positively VETOED this idea as it would involve major surgery to the
dash (a large speaker hole would have to be cut in this compartment). The
speaker-enclosure-behind-the-dash idea might actually be the best as there
appears to be a certain amount of room back there, and it does not involve
hacking my dash up.

MOSFET