The knob in the middle is a rotary action on/off electrical switch. It allows you to stop the electric wipers in mid operation so that you can maintain or replace them.
When you put the normal wiper switch into the "off" position, the wipers automatically park themselves down in the bay under the wiper door and the door closes.
So, to work on the wiper blades, switch on wipers as normal and then use that rotary switch to interrupt the wipers.
Don't forget to switch it back again when finished otherwise you'll think your wipers have gone on the fritz next time you need to use them.
One vacuum switch manually opens the wiper door, the other pops up the headlights. Naturally, the vacuum system needs to have vacuum in it at the time for either switch to be effective.
