wiring problem

i have a 1970 vw bug and i took out my speedo and put a 5 inch tac there and the bulb that is suppose to charge the battery is not hooked up any more. the green wire that comes off the alternator is the one that charges the battery and i dont know what color wire i should hook the green wire to to make it work. the alternator red wire reads at 12.4 and the battery reads at 12.4 when the car is not running. when it does run they both run at 12.9. can anyone help me with this. thanx if you can..

Kenny