Before you resort to cutting the dash, you might try removing the instrumewnt cluster to gain better access to the rear of the switch (I know, I know, pulling the cluster is a knuckle buster, too, but your bleeding fingers will heal themselves, the metal dash won't )
BTW, a '65 Caliente was my first car - LOTs of Mustang parts (guages, sway bars, brakes, rear end, etc., etc.) can be swapped between them. Just about any mechanical part (save for the Export brace) from a Mustang will fit the Comet.
regards,
Jeff