Is this the gauge you are installing ?
If so, go to http://www.summitracing.com/parts/SUN-CP8204/ and in the upper right corner there is a PDF file you can open and print with the instructions.
Oh boy, I did this....so I should know, but it was 25 years ago!
Take a look at the wiring diagrams on page 530 and 531 here:
http://www.65gs.com/65%20Chassis%20Service%20Manual/1965%20BUICK%20SPECIAL%20CHASSIS%20SERVICE%20MANUALindex.html(I'm looking at a 66 GS wiring diagram, but the sb posted one is close)
There's a 10 gauge red wire between the alternator and horn relay/junction block.
Another red 10 gauge wire runs thu the firewall where it supplies the fuse box, light switch, and ignition switch.
So this is similar to pic #2 of the Sunpro instructions, except our junction block takes the place of the Battery in the pic.
According to this, you must remove both 10ga wires from the junction block, and splice the ammeter in there.
