How can I replace a fresh/recirculating air switch on 1999 Vauxhall Astra?

You do know these switches will not work when the direction knob is set to demist? The switches are mounted on the printed circuit board and are not replaceable. Try moving the direction knob and see if it works then, there is a micro switch fitted to the direction knob which shuts the power off to the recirculation switch but that's not replaceable either.
I do have some PCBs from these units but there are 2 types and you need to identify the correct type for your wiring.

Oddjob
September 2008