Red: 7.5 Volts

Realized that if I was short on voltage with the engine running and the lights on, then why not add a couple extra?  Seemed like the low horn volume when running was simply due to the low volts.

So rigged up an extra D battery to clip into the existing harness.  Tested ok, so then soldered everything together.

Test run showed a much better horn volume, though still not super loud...