This answer is GMC based, but I'd bet your life it is the same on Chevys. I'm pretty sure the electric wiper setup I put into my '57 GMC back in the 90s was from a TF Chevy.

The stock knob on the dashboard just moves a cable, as does the vacuum wiper knob. Unfortunately the two cables are not the same. The cable for the electric wipers operates two sets of contacts INSIDE the wiper motor assembly. You can find the GMC info starting on THIS page and continuing for 5 pages.

A guy that was the electrical equivalent of mechanical genius Hot Rod Lincoln could open up the electric wiper mechanism, remove the moving contact for high and low speed and wire the aftermarket switch in to replace them. I'd probably screw that job up and no longer have an operational electric wiper motor.

'57 GMC 102, Original 347 V8, HydraMatic, 3.08 rear gear, added A/C, disk front brakes, HEI, AFB carb, '98 Honda Black Currant paint. T-boned and totaled 10/12
'52 GMC 152 Stake Bed, Original 228, SM420, added A/C, HEI, disk front brakes, '67 Chev 3.55 rear gear. Gets used as a real truck.