1990 350 Brougham (The cruiser), 1988 Mark VII LSC (The DD)
The hi/lo switch does not turn anything off, it simply switches between the two. The parts that go into the computers should only be signals. They tell the computer when the lights are on (for things like auto dimming mirrors) but does not actually control the lights. The twilight sentinel is the only thing that controls the lights. Have you checked the grounds at all? If you could upload a picture of the wiring diagram I could have a look and give you some potential problem areas. Its one of the few things I am good at.