Ok here is one for the experts, i have a 2006 joyner sandviper / Dune buggy 3cylinder engine full size alternator 12v system. I recently installed a JVC headunit and some speakers. The headunit plays fine and everything works perfectly...with the engine off

when i start the engine the unit display just blinks, i have the constant power right to the battery, and the other 12v to a key on power, unit is grounded to frame. now i belive i figured out that not running the battery voltage is 12.25 running it is 14.40. so that trips up the head unit... question is how do i regulate the voltage to stay in the 12v range so the headunit will play with the engine running?
THANKS IN ADVANCE