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Super Moderator
1966 Crown Coupe, 2016 200 S AWD, 1962 Lark Daytona V8.
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I saw this in the J-car section a few days ago and was going to pull the service manual and sorry I forgot. By interior lights, you mean the cluster illumination/VF display lights? Yes, full display brightness can be a real eye-burner at night.
The dash illumination incandescent bulbs are grounded all the time and receive a varying '+' voltage from the BCM on an Or/Yl wire.
The VF displays (PRND3L and odometer) receives '+' power all the time on a Rd/Vt wire to the cluster and the ground side of the bulbs are dimmed by the cluster circuitry itself depending on a bussed in request on the CCD bus by what the BCM sees from the desired dimmer control setting.
Something may still be wrong with the dimmer control inside the headlamp switch. When the Parking or Headlamps are turned on, the 12 volts internally powers one side of the dimmer rheostat. The other side of the dimmer rheostat is grounded to the left cowl area sheetmetal and the variable wiper contact goes to the BCM on a Rd wire. It's like a voltage divider/variable resistor.
By varying the control with the Parking lamps on, you should see voltage vary between ~2 to 12 volts if you can back-probe this Rd wire at the switch. If the dimmer has lost its ground and the wiper is staying at 12 volts, I would think that the illumination and VF displays would remain at full brightness whenever the exterior lights are turned on.
I could not find any pinout or wiring diagram images on Google to help here and I know how much a picture can be worth a thousand words. Good Luck.
 

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Super Moderator
1966 Crown Coupe, 2016 200 S AWD, 1962 Lark Daytona V8.
Joined
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23,878 Posts
I may have it backwards on the Rd dimmer wire. Grounded may be full brightness. 12 volts may be full dim.
It should not always be ground. See if the short to ground may be at the switch terminal side or on the Rd wire side?
 

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Super Moderator
1966 Crown Coupe, 2016 200 S AWD, 1962 Lark Daytona V8.
Joined
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23,878 Posts
It sure is beginning to sound like a failed multifunction switch.
 

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Super Moderator
1966 Crown Coupe, 2016 200 S AWD, 1962 Lark Daytona V8.
Joined
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23,878 Posts
It is the same part between any Cirrus, Stratus or Breeze JA or JX convertible until 2000. The only difference is with or without foglamps.
 
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