Reviving this thread once again. I think I figured out where one mystery vacuum line goes to, and it must be my MAP sensor. It's a line that T's off of the one that also 'feeds' the heater box. But, rather than being mounted on the firewall, or the passenger fender, my line goes straight into a wiring harness, which then runs to under the fan housing on the passenger side firewall, then loops back around towards the power distribution box. And ... never comes back out again, that I am able to trace. So, unless I undo all the (original looking) insulation wrapping of the harness that it goes into, it doesn't look like I can figure out where it terminates. Which I think it does, somewhere along the line, without connecting to an actual MAP sensor. So although my truck was sold and stayed in California since 1990, it does not seem like it ever had a MAP sensor, although the vacuum line to enable it, does seem to be in there.