I wouldnt use the AA's all youll likely do is flatten them, if the battery is lighting the neutral/oil lamps its good enough to work with!
I have to make an assumption the lighting works like my DT175, thats the key in 2nd position and left handlebar switch to light on posn, this brings on the tail light and a small pilot light in the headlamp bowl...the main headlamp only works from the engine running
The brake light (yellow wire) should become live when the brake is pressed with the keyswitch in either of the first two positions