Sorry to jump in late on this one - did you get it figured out yet?
I have a couple of thoughts on this if it'll help:
I think you could wire the "from the rheostat" terminal, to the instrument light switch that's under the dash. Just leave the under-dash switch on and do the controlling of the dash lights with the rheostat on your new switch.
The stop light switch is "hot" all the time and isn't switched off and on. The original wiring diagram shows a fuse on the light switch that feeds the stop light switch. Does your new light switch have a fuse on it - or - did your wiring harness come with any fuse holders or in-line fuses? If so, check and see if one of them is supposed to wire to your brake light switch.
Hope this helps.
