This is the setup I'm using, and it seems to work good so far. Basically I've put in a shunt through the fender junction block so that my original style amp gauge isn't getting the full load. (For those using an aftermarket style gauge that can handle full loads, there is a wiring diagram at
http://mightymo.org/Proj_OneWire.html that should work.)
The "Splice 2" in the diagram is part of my wiring harness, and that's the name that it has on the wiring diagram in my shop manual -- it isn't a splice that I put in myself.
Any comments from those with more electrical experience than me are greatly welcomed -- I'm sure there are a lot of people who know more than I do about this electrical stuff, so let me know if I'm going wrong somewhere!