[NiHaoMike]

One approach is to find the sense point of the voltage regulator circuit and add a resistor (value calculated based on existing resistors) to it. Connect it to 12v to turn it off (the regulator will sense higher voltage and reduce the duty cycle of the pass transistors) and connect it to ground to force it on (the regulator will sense lower voltage and increase the duty cycle of the pass transistors). A small relay can ground the line when the brake lights are on for a little regenerative braking. A switch with a resistor or pilot light bulb in series (to prevent a shoot-through short circuit) can connect the line to 12v to disable the alternator, but not disable regenerative braking. (Alternatively, the common of the regenerative braking relay can connect to the line, with the normally open to ground and the normally closed directly to the alternator disable switch.) I'm not sure if the regulator would become unstable from this, but a datasheet can be checked for information on design.