The line going to the "switch" is most likely the EVAP line. It is going to the control valve and solenoid. The line going to the firewall is what provides vacuum to the HAVC & parking brake release, but I don't recall it going through any switch before entering the firewall. Has anyone messed with the vacuum lines? The switching from floor to dash is done by the programmer aka ACM (Air Conditioning Control Module) which is down line (under the dash) from the firewall. On the pre 2000 Devilles heat goes to the floor and A/C goes to the vents. Since putting direct vacuum to the firewall line solves the problem, it obviously lies in that "switch", though I can't figure out what it is or why it is there.