there should be 2 relay boxes under the front seat, the one with the red connector is the fuel pump relay, the other one, i believe, is the lighting relay. been a while since i had to mess with one, but i believe its a (roughly) inch and a half by 2 inch by maybe a half inch black box .

next time it happens check and see if you lose the tail lights too. if you do, it's not the relay b/c it only controls the headlight if i remember right.