Ok - if the automatic lights are working properly as you described, I would go back to suspecting the headlight switch/dimmer. First, I would check to be sure that the Rear Parking Lamps illuminate to full brilliance. (The voltage that goes to the dimmer switch originates from that same +12 volt DC circuit fuse according to the schematic). If not, check the fuse - my 2000 LS fuse for this ciircuit is located in the Passenger Side Instrument Panel Fuse block. Check your manual for the location - might be the same - I'm not sure. If the Rear Parking Lamps Illuminate correctly then I would proceed to probe the circuit as I described earlier for voltage reaching and leaving the actual headlight dimmer switch. That will tell you whats going on for sure. If you dont have a voltmeter or are not comfortable probing the circuit you could just try replacing the switch/dimmer module. I'd try to get a used one or one from a junkyard in that case