I used the 3-pin auxiliary lighting plug (key-on). The key-on +ve and -ve also powers my LED driving lights and the relay trigger signals for my ABS defeat. Strictly speaking it's probably best to draw direct from the battery except that some of the meters draw current full time...might suck your battery dry over extended non-running time.
If you have no intention of running anything else, it might be easiest to tap into the cigar lighter wires (a 3A circuit) since it's right there.
If you're looking for a 3-pin plug set, let me know. I've got about 15 sets left and I can mail you one in a letter envelope. I've also got the right crimping tool so I could do the leads for you before sending.