A 9 volt transistor radio battery and two AA will not be able to supply the needed current for the regulator and even one gauge. Each gauge is about 20 ohms and a middle scale sending unit is 24 ohms that's 44 ohms total. With a 12 volt source you have 12volts/ 44 ohms = .27 amps. That's quite a bit more than you can get out of a 9 volt transistor radio battery. Eight 1 1/2 volt "D" cells would work much better.