1. First set your voltmeter to the continuity setting (should beep when you touch the red and black probes together, telling you they are shorted).
2. Find a ground on the car somewhere, like a bolt under the dash, or a metal cross member.
3. Put the black probe on that ground, and touch the back of each exposed wire connector in the mirror harness with the red probe.
4. For each wire that beeps, that means that wire is grounded. Turn the ignition to the on position, then back to off, and make sure it beeps regardless of ignition position.
5. Put the voltmeter into VDC mode, to measure DC volts, turn the ignition to the on position.
6. Put the black probe on the ground wire found in #4.
7. Touch each of the wires in that harness with the red probe, and note which one(s) gives you +12-14 volts, turn the ignition off to verify that the volt readings go down to zero or close to zero.
8. That should define which two ports in that harness into which you would plug the MirrorTap, the black Prong into #4 above, and the Red to #7.