If your TAIL lamps are ON, probably faulty is the connection from the TAIL and the HED lights, at the light switch, control if connections are right and strongly connected.
Sorry haven't the 100/4 Wire diagram I can't offer more aid... so
If with this first control you NOT fix the problem, the better instrument for this investigation is a simple 12 V lamp single filament, with the bulb holder, connect to it TWO long (120 cm) wires, possibly one Red and one Black
(ground depend if your car is POSITIVE or NEGATIVE grounded- normally AH 100 are POSITIVE ground- so RED must be connected to bulb holder ground-
if your car are modified turn to the correct color)
connect the Ground of the lamp to the ground of the car (this contact be fastened and fixed in the engine bay) and connect the battery and the Ignition switch
NOW with the free wire touch the contact beginning from the fuse box contacts-when electricity is present, lamp go ON - and continue the investigation