With key "on" confirm that you have fire at the blue wire alt field terminal and to the top blue wire terminal in the triangle connector in the flat reg. Confirm the green wire from the other alt field terminal to the "side" terminal in the triangle connector of the flat reg has continuity end to end (likely it does). seperate/check/clean the bulkhead would be my first move. alt is OK, new reg, reg is likely grounded well since you replaced it & bolted down a new one. all that is left is the above wiring checks for the open