I agree with genixias comments.

I assume the +12V line top right is input fed from car battery - ie anything from 9-15V from when you crank the starter to when the alternator is charging fully...

So that means that you should have a regulator with lower output than 12V, and that you can adjust its output voltage somewhat by using the pot.

A SPDT switch doesn't have one "on" position - it has two. In one position the center connector is connected to one of the other connectors and in the other position the center connector is attached to the other of the "other" connectors. Note that "center" refers to logic, not neccessary physical position.

So going strictly by how the switch is drawn you should have either the "from original harness" or the adjusted voltage from the regulator goint to the common "MAP" wire depending on switch position.

Depending on the type of signal that is on the "from original harness" it may be impossible to distinguish switch position easily. Simplest way to get a switch position indicator LED going is probably to change the switch to a DPDT switch instead. In practice it is 2 parallell, commonly activated but electrically isolated SPDT switches - ie has 2x3 connectors.

Use one of the sets to switch the regulator output/from original harness/MAP wires and use the other set to switch power to the LED. A typical LED will in car use work OK with a 1kOhm series resistor. The generic formula for series resistor is (V_supply - V_ledforward) / I_ledforward.
ie the required voltage drop over the resistor (supply voltage minus the forward voltage drop over the LED) divided by the LEDs current draw at the voltage in question. Supply voltage is a bit tricky in a car - as long as you don't use the max parameters for the LED, 13V is a useful average considering engine off or on. Also check power dissipation in the resistor - P=(V_supply - V_ledforward) * I_ledforward. Here it's a good idea to use 14-14.4V as supply voltage (worst case...). Then get suitably sized resistor.

/Michael