Jim, that particular line never gets vacuum from the pump directly, its the routing is always like you said from the vacuum switch on the dash. The difference is actually the switch itself, if I understand you correct your '88 has a separate switch for that door, whereas '89-up trucks have the switch built in with the vent-selector lever, as in it automatically closes the door when you choose "off" or "max A/C", and keeps the door open in all other lever positions. In my '90 truck I wired in a separate vacuum solenoid and installed a stand-alone switch for it so I can open and close the air door manually as I see fit, kinda like how you can with your factory setup