when i did my two daily driver cars, i installed it inline on the power side between the brake switch and transmission connector at the trans for the lock-up torque converter. when full vacuum is applied to the vacuum switch, the internal switch will close and complete the circuit to the trans. during part throttle ( like when your climbing a hill) the vacuum from the engine will drop causing the vacuum switch to open, which disengages the torque converter. once you crest the hill your engine vacuum will rise, applying enough vacuum to the switch, and the converter will lock up again.
wide open throttle works the same way.
this vacuum switch is just an add on. i like using it because it makes the system more automatic feeling, like the factory ecm system. some people use a dash switch, but then you have to remember when to turn it on or off. not a big deal, but i like my automatic trans to be be automatic.
sorry, no pics. i switched to a 6spd manual 4 years ago. new is always best, but you may need to scrounge for the electrical connector.
vacuum switches were available on the 1980 gm pre-computer cars and on early 80's chevy trucks and vans equipped with 700r4 trans.
The switch itself you can find on Summit, and I'm sure you can find it on Rock Auto or even your local parts store. As for the connector, that I'm unsure of as well. I checked the suggested parts section on Summit, but it came up with nothing. I'm sure if you really wanted to, you could simply hard wire the switch and do your best to insulate it from heat to prevent it from breaking connection. As for what should go where, that information would undoubtedly be in a repair manual. Likely it would be wired to something that is on whenever the ignition is on, like the oil pressure switch, or a electric choke.