Hi Creg,
mine is (was, damn!) attach to the main ignition red wire, the big one if
you remove the steering column (plastic cover, 2 screws under this one!).

Any other places i tried gave to many variation on the needle.

The best way is direct to the battery with a inline fuse of a few amps.
you will get about 13V if attach to the column, 13,4 if direct to the
battery.(without any electic componant working)

Also, if attach to the red wire under the column, you will see all drain
cause by the eclectic accessories (fan blower, light, wiper), when all
those are working at the same time you will have a 12v to 12.5v reading.

I remove my arternator belt just for fun and the reading was quite below 11
volt.

Cheers, Ben