I'm planning on doing the same thing. I haven't done the under-dash research, but it was no big deal at all to run a new wire through the rubber pass-through in the firewall. In fact, there's a little 1/8" extra unused hole in it just perfect for the job. I upgraded the headlight harness a while ago and installed a fuse sub-panel in the engine bay, so I have no need to run to the interior panel - I'll just run to a spare slot on my sub-panel.
As for the switch, believe it or not, you can get an OEM Toyota fog light switch on
Amazon for $12.
The bezel is the right size and it looks 100% factory installed, but it helps to trim back the 4 plastic retaining springs. They're just a bit too strong, and forcing them in might crack your decades-old plastic. IIRC, the switch is rated at 20A, so if you run a low-draw system like LEDs, you may not even need a relay.