[Dnsjo]

After more reading and fiddling.

You have your choice of immediate activation or timers for later activation.

Engine running: immediate activation grayed out.
Engine off and ignition left on: immediate Activation command available. Timers available.
Ignition off: nothing available (duh!)
Ignition turned back on, engine off: immediate Activation grayed out. Timers still available.

It seems you can only activate the PC vent immediately after shutting off the engine, for example if you're just making a brief stop and want the vent left running. If you turn off the ignition and power down everything, you can't simply turn the ignition on to activate the PC vent. You would have to start and stop the engine to again have it available, or as suggested below, set a timer to have it come on whenever you wished.

The vent runs 30 minutes and must be set again after 24 hours.