Well, there's the glove box, under the seat or in the overhead (since there's room and you can run the wires up the "A" pillar).

I might take choice "C" so you can still get to it for channel/volume control, etc. Might want to consider fabbing a shelf to go over the windshield. Should leave you with enough space for several electronic goodies.

87 Montero, bought new, by me -
88/89 intercooled Starion Turbo engine-
with awesome audio.