That circuit definitely changes constant 12 volts into a pulse, but I'm missing a couple of things.

First, if you had bypassed the first relay and just put 12 volts from the Anthem into the capacitor, then taken the - side of the capacitor and run it to the Proceed, you'd have a pulse. In that case the second relay would only be there to bleed the cap, and in my case I put a resistor from + of the cap to ground to discharge it.

But if I understand correctly, you need one pulse to turn on the Proceed and another identical pulse to turn it off. I don't see how this does that.

I like the capacitor charge idea, though -- I used to have one of those 70 horsepower Austin Powers Anglias (1970) and I put a BEEEG horn on it, run through a circuit similar to this. Hit the horn once, and you got the anemic Anglia horn while charging a capacitor. Hit the horn again within three seconds, you got an air horn!