I did some more reading, and this is what I found:
"Your transmission (4L80e) is already 12 volts. The HMMWV A2 has a dual output alternator that charges both 12 and 24 volt circuits. Your transmission taps off one battery and sends 12 volts to a bi-voltage relay next to the brake switch. When keyed on-the 24 volt relay then powers all 12 volt circuits. The transmission computer is fed through the two vehicle speed sensors (VSS) that are mounted on the transmission case, and sees input from these two sensors, a 12 volt powered brake switch, and throttle position sensor." (
12v 24v dilema [Archive] - Pirate4x4.Com Bulletin Board)
"The REDARC series of Charge Equalisers (CE) are designed to operate 12V equipment from a 24V system, without the need to buy another 12V battery. The CE Charge Equaliser allows you to 'tap' into the 12V from the lower battery of your 24V system without flattening the lower battery or overcharging the upper battery. REDARC charge equalisers are available in 3, 10, 20, 30, 40 and 60 Amp models. The CE Charge Equaliser is a relatively inexpensive, simple way of getting pure DC battery voltage to run 12V equipment from a 24V system." (
CE Series Charge Equaliser | Redarc Electronics)
"A major decision for those converting a military vehicle to a fire truck concerns the electrical system. The military vehicle will be 24 volt and most fire appliances require 12 volt. The HMMWV’s are no exception. Project 56C guides you by showing how to add a separate 12 volt electric system, keeping the 24 volt one for the vehicle and using the 12 volt for add-ons. Appendix A provides a schematic for wiring the system. (
http://www.roscommonequipmentcenter.com/projects/rec56c.pdf)