My sons 1971 Pontiac Lemans Sport problem solving

Trying to solve last few issues for my son on his 71 Lemans Sport. We have replaced the relay in the heater wiring buy still only get one setting and only if i put power to the back side or jump the wires. He has factory Heat/AC and has 4 posts but only three wires in them. There is a wire loose not sure if it should go to the relay or ? LOOKING FOR  A WIRING DIAGRAM for the engine side if anyone has one please. And LAST he has no control to switch between levels because the vacuum line comes through the firewall into the  motor side and goes nowhere, looking for the vaccum routing for the 350 4 barrel manifold. where does it hook to? Thank you all. If we can get at least the heat going he can drive it more, other option is to sell off the stock and go Vintage air or such.