If your car was originally a two-speed wiper car you should already have the harness unless it was removed. If your car was not a two-speed wiper car, you will need to get the extra harness and you will need to drill an approximately 1 inch hole in the firewall for it to pass through (at least to do it the way the factory did).
The special two-speed harness plugs into the wiper motor, runs through the firewall and plugs into the switch. The underdash harness hot wire plugs into the two-speed harness at the switch and the firewall harness plugs into the two-speed harness at the wiper motor.
To be clear: as far as the two-speed wiper switch and motor is concerned, there are no differences in the underdash harness or the firewall harness compared to the one-speed set up. The whole difference is made up by the special two-speed harness and the two-speed switch.
This picture may help with the exterior look of things. You can see the two-speed harness coming through the firewall and running to the motor. The wire hanging loose from the motor accepts the firewall harness power wire. Mine also has windshield washers so I may have a few more wires than you.:
NCOA member #220
ACES member #1670
Team Chevelle #998
YesterYears Chevys #52949
Rich's 1965 Malibu SS Restoration Project Progress Page
1:18 scale diecast car Detailing by Scooter