Just wanted to let everyone know that I finally got that volume dial installed! I was about to created the circuit you outlined by Apocalypse when I came across another less invasive option.
Which was to use this specialty item called and L-Pad Attenuator, intended for us in auto audio:
http://www.parts-express.com/speaker-l-pad-attenuator-50w-stereo-1-shaft-8-ohm--260-260
This thing is simply wired in-line with the speaker wires & ground and while it doesn't cover the full range of volume, you can definitely get a range of adjustment that goes from objectively 'quiet' to 'pretty loud'.
The wiring is simple but diagrams included with the products are a little confusing.
Ignoring the numbers on the actual pins, I'll label the the pins here with letters, arranged like this when looking down at the part with the rotary shaft pointing towards you:
A B C
D E F
C and F both get wired into the ground (solid black wire in my cab). Individually, not passing through. In my case I wired them together, then sent one wire back into the black speaker ground wire.
Then the audio wire going to one of speakers (doesn't matter which, they're black w/ red stripe and white w/ red stripe in my cab) gets connected to B, and the corresponding wire going to board gets connected to A.
Same for the other speaker wire through D and E. Where E goes to the speaker, and D goes to the board.
Sounds confusing but follow that carefully and it should work. Here's a pic from my rigging:
As far as mounting the thing, I ended up drilling a hole in the control panel and placing the dial in the far upper, right corner. Mirroring the placement of the P1 Start button on the left side. Looks classy and of-a-piece. I had to chisel out about at 1/4 inch from the particle board to the part would clear without bending the shaft sideways a little.
The knob that comes with the part is hideous of course, you can go to any radio shack that has electrical parts and pick up a bunch of replacement knob options, the sizing is pretty standard.
Enjoy!
