Sounds like a ground issue , resistance . Find a wiring diagram here http://www.bbbind.com/free_tsb.html Enter vehicle info. year , make , model , engine . Under system click on body & accessories . Subsystem click on instrumentaion . Click the search button . click the link . Then watch this video on voltage drop testing Mastering Voltage Drop Testing with Pete Meier and Jerry Truglia
Electric Testing Techniques You Need to Know Pull the instrument
cluster to where you can get to the connector plugged into the back an do loaded circuit voltage testing .
Whenever the headlamps or parking lamps are on voltage is supplied to the manually operated dimming level control switch in instrument cluster through the parking lamp feed circuit 9. The output of the dimming level control switch is then converted into a serial data message that is sent through serial data circuit 1036 to the data link connector (DLC) Splice Pack #206. The Body Control Module (BCM) uses this input to control the dimming of the brake transmission shift lock control PNRDL indicator, the HVAC control assembly (C60 or CJ2), and the electronic traction suspension control switch lamp. The brake transmission shift lock control PNRDL indicator is supplied battery voltage at all times through circuit 240. The electronic traction suspension control switch lamp is supplied voltage through the parking lamp feed circuit 9 whenever the headlamps or parking lamps are on. The BCM provides a pulse width modulated (PWM) ground circuit to vary the intensity of the brake transmission shift lock control PNRDL indicator, and the electronic traction suspension control switch lamp. The HVAC control assembly (C60 or CJ2) receives a pulse width modulated (PWM) voltage supply from the BCM to control the intensity of its display. The HVAC control assembly (C60 or CJ2) is grounded through circuit 150 to Splice Pack #202 to G202.
The class 2 data message which is sent out by the instrument cluster is also used by the LH and RH Door Control Modules (DCM) to control the dimming of the LH door switch illumination and the RH door switch illumination. The DCMs supply the 12 volt feed circuit to the door switch illumination lamps and provide a pulse width modulated (PWM) ground circuit to vary the intensity of the illumination lamps
Repair or replace the wiring harness connecting the instrument panel dimmer rheostat to the instrument cluster.
Is the repair complete?
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Go to Step 6
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4
Replace the instrument panel dimmer rheostat. Refer to Instrument Panel Lamp Dimmer Switch Replacement .
Is the interior lights dimming function operating normally?
Repair or replace the wiring harness connecting the instrument panel dimmer rheostat to the instrument cluster.
Is the repair complete?
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Go to Step 6
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4
Replace the instrument panel dimmer rheostat. Refer to Instrument Panel Lamp Dimmer Switch Replacement .
Is the interior lights dimming function operating normally?
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