In the Apollo 17 Wikipedia article, the coordinates of the descent stage are given: 20.19080°N 30.77168°E. What are the specifics of the Moon's coordinate system? Where is 0°?
0° is the line of longitude that contains the point on the surface closest to Earth. In other words, (0,0) is at about the center of the face of the Moon we see.
The details are a little more complicated, since the "closest point" is really a mean position. This document has much more information on the coordinate systems and other lunar standards.
Latitude: 3° 12' 43.2" South Longitude: 5° 12' 39.6" West
All coordinates are defined relative to this location, so there's no need to rely on a magnetic field, although the Moon does have one. For more information, please read this RAND corporation report of 1987, and the fundamental IAU/IAG paper.