Let's approach this from the other end. On the intake manifold above the ALDA there's a pressure switch with one wire attached to it and just beside it is a nipple extending from the intake manifold. There should be a hose from this nipple to the vacuum switch on the firewall. That bit that goes into a bundle is the electrical hook up for the vacuum switch.

Sixto
91 300SE
87 300SDL
83 300SD