On my plain jane XtraCab the mirror/light combo mounts via three screws.
On the 4Runner that I am getting the map light from it appears to be the same arrangement with the map light mounting aft of the mirror/light combo, via another three screws. I am hoping that what the map light mounts to is a bracket that is removable. I'll know tomorrow evening when I next visit the carcass.

'88 EFI Xtracab, 32 x 11.50 Buckshots, AZ bed, Turbo drivetrain.