Mine's 10 years later, but just like Rich said it's cable operated through an unused radio hole. See the big white "Muncie" knob at:
http://flyingmelons.com/JDM/1958Chevy/DashCenter01.jpg be advised this is not "factory" but only where the aftermarket builder put it back-in-the-day.
That white knob engages the PTO, the "T" knob to the right is throttle and the actual pump activation lever is outside at the back of the cab.
