I bought a cool DC voltage readout, that I have physically installed into the dashboard and now I want it to work only when the truck is running, otherwise not.

It has two wires, black (-)/ground and red (+). Where should I connect the red one ?

Also removed the old radio, it wasn't that had, I just did as someone here suggested and the radio is connected with these clips from the back, so after the plastic dashboard is loose, you use a screwdriver and push those in and it comes out cleanly.

 

The only way I know is to have it wired by tapping in line using the 'batt' lead from the alternator.However,I think its probably intended to read battery voltage,by connecting red to any 'hot' terminal,and black to any ground.
....As a second thought,I'm going back to when I wired in an ammeter.I'm not an EE,but you may be able to have a relay send voltage to your meter if it's energized by the ignition switch being turned on.