Fullerton's tower is NOT closed. I don't know where you're getting your information.
Umm.. actually, he is right. KFUL does indeed close. From AirNav
FULLERTON GROUND: 121.8 [0700-2100]
FULLERTON TOWER: 119.1 [0700-2100]
Bold for emphasis. KFUL ground and Tower both terminate operations at 9pm daily. when the tower closes, it appears that the Tower's frequency is used for CTAF. Best way for you to find out is to give a call to their ATIS after 9pm, and see what you get. I'll bet you anything, the tower is closed.