Set the pot somewhere in the middle of its range

Apply 12V to input (and ground) - battery or AC/DC power adaptor.

If nothing goes up in smoke it's a fairly good sign

Put multimeter on the closest higher DC Volt setting (might be shown as V=) than the voltage you intend to measure. In this case a setting of like 20VDC (ranges vary betwen meters, but you get the idea) is good.

If you measure between the the output and adjust pins on the regulator (or any thing directly connected to them) you should have 1.25V (the internal adjustment voltage used by the regulator)

If you measure between ground and the MAP wire (with switch in appropriate position) you should get a reading somewhere around 5V. Adjust the pot and see if/how the reading changes. Adjust until you have a reading you think your car will be happy/at least functional with...

Temporarily hook up another resistor (100-200 Ohm or so) in parallell with the 520 Ohm one
(adds a small load) and check that the voltage remains the same (the regulator regulates properly).

Flip the switch and you should get 0V (unless you have the "from original wiring" wire hooked up to something)