[rpw6667]

67 GTO NO LIGHTS
Headlights work
No parking lights/no 4 way flasher/no taillights/map reading lights dont work
Open door and courtesy lights under dash work

Checked fuses........OK
Light switch has 12 volts on red and blue wires, no voltage on purple/brown, or white

Any help would be appreciated, thanks

[OG68]

Normal Configuration:

Red - 12 volt Battery feed from fuse panel
Lt Blue - 12 volts from HDLT switch to dimmer switch to HDLTs
White - Ground wire for dome or map lights, should show 10-12 volts with doors closed (12 volts feeding through dome/map light bulb). 0 volts when door is open or HDLT switch turned all the way to the left
Purple - 12 volts from HDLT switch to front parking lights
Brown - 12 volt Battery power from Tail light fuse to HDLT sw to tail lights

Questions.
1. Map lights are over the doors and replace the dome light and consists of a combination Map/Interior Light. Is this what you have? Is it the actual map light that doesn't work or the Interior light section?
2. Do the Brake lights work?

TAIL LIGHTS / PARKING LTS
My first thought is the headlight switch is bad. If the fuses are good as you say then the only other possibility is the switch. (Normally)

MAP LIGHTS
Check Bulbs
Check for power in Map Light socket (Orange wire, door open)
If you have map lights over the doors the wire connection is usually over the drivers side rear wheel well inside the trunk. See if you have an Orange & White plug from the rear twilight harness there. It should connect to the Orange/White map light harness. It has the same fuse as the under dash courtesy lights (Normally)
Check rear harness plug under drivers side carpet if the everything else checks out.

FOUR WAY FLASHERS
The four way flashers are fed from 12 V Battery power through the flasher on the fuse panel to the turn signal switch.. Easy check is to replace the flasher can first then the brown wire to the signal switch. Power will fluctuate from 0 to 12 volts. Connector is on the lower steering column.

I say "Normally" as this is the factory configuration, previous owner modifications can change everything.