Control panel is so dim I cannot see anything on display. No error messages were received. Panel has gradually darkened and is totally off now.

Interior Lights Dimming Circuit Description
VF Displays
When the ignition switch is in the RUN position, the instrument cluster activates its VF display. The instrument cluster also coordinates the display illumination for the radio (IRC), the cellular telephone (PHN), and the climate control Panel (CCP) anytime the ignition switch is in the RUN position.
Incandescent Dimming 1 (INC DIM 1)
The INC DIM 1 consists of lamps powered by CKT 8 (GRY). These are referred to as the park lamp backlighting because they are only active if the park lamps are on and the ambient light sensor indicates darkness.
Incandescent Dimming 2 (INC DIM 2)
The INC DIM 2 output is used as a general interior light source and an illuminated entry feature that provides interior lighting. The INC DIM 2 consists of lamps powered by CKT 1244 (BRN). When one or more of the following conditions occur, the INC DIM 2 lamps will be active:
• The interior lamps switch is active.
• The park lamps are on and the ambient light sensor indicates darkness
• The ignition switch is in the RUN position and the ambient light sensor indicates darkness
• A Class 2 message is received from the body control module (PZM) indicating that the courtesy lamps are active.
Dimming
The instrument cluster controls all of the interior dimming functions. The instrument cluster receives an ambient light level input from the ambient light sensor and determines whether it is light or dark. If the ambient light sensor is inoperative, the instrument cluster will assume a condition of darkness. Dimming of the VF displays is only allowed when the ignition switch is in the RUN position, the park lamps are on, and the ambient light sensor indicates darkness. If all conditions for the VF dimming are met, the instrument cluster uses the input from the potentiometer in the headlamp switch to determine the level of dimming. The dimming level is sent to the radio (IRC), the cellular telephone (PHN), and the climate control panel (CCP) via a Class 2 message. These modules then adjust the dimming level of their VF display. The instrument cluster provides dimming control of the incandescent lighting through the INC DIM 1 and INC DIM 2 outputs. The instrument cluster provides dimming control of the electronic PRNDL LED through CKT 1382 (PPL/WHT). The dimming level will very based on the potentiometer in the headlamp switch whenever the parklamps are on and the ambient light sensor indicates darkness, regardless of the position of the ignition switch.


1998 Cadillac DeVille ' DeVille, Eldorado (VIN E/K) Service Manual ' Body and Accessories ' Lighting Systems ' Diagnostic Information and Procedures ' Document ID: 489166
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Interior Lights Dimming System Check
Step
Action
Normal Result(s)
Abnormal Result(s)*
1
Turn the ignition switch to the OFF position.
Place the interior lamps switch in the ON position.
All incandescent lighting on the INC DIM D2 output turns ON. INC DIM 2 includes all of the lighting on CKT 1244 (BRN), which is the backlighting for the headlamp switch and the floodlighting for the vehicle doors.
Incandescent Lamps Inoperative
2
Place the interior lamps switch in the OFF position.
Turn the ignition switch to the RUN position.
INC DIM 2 lighting turns OFF.
All of the VF displays light. VF displays include the following components (if equipped):
• The radio (IRC)
• The cellular telephone (PHN)
• The driver information center (DIC)
• The instrument cluster (IPC)
• The climate control panel (CCP)
• Vacuum Fluorescent Displays Inoperative - All
• Incandescent Lamps Always On
3
Cover the ambient light sensor with the twilight OFF.
Place the headlamp switch in the PARK position.
The park lamps turn ON.
All of the VF displays dim in order to match the current dimming control level. All of the incandescent lighting for INC DIM 1 and INC DIM 2 light at the current dimming control level. INC DIM 1 includes all of the park lamp backlighting on CKT 8 (GRY) which is composed of the following components:
• The radio (IRC)
• The front cigar lighter
• The steering wheel controls
• The driver information center (DIC)
• The climate control panel (CCP)
• The instrument cluster (IPC)
• The heated seat switches
• The auxiliary blower switches
• The ash tray lamps
• The PRNDL lamp
Exterior Lights System Check
4
Rotate the headlamp switch dimming control in both directions.
All of the VF displays and all of the incandescent lighting dim and brighten in response to the headlamp switch dimming control.
Vacuum Fluorescent Displays Do Not Dim with Instrument Panel Dimmer Switch
* Refer to the appropriate symptom diagnostic table for the applicable abnormal result.

The capacitor on the back of the display is failing. Heat warms up the gel inside it. The capacitor can be replaced by an electronics tech or you can buy a new control board display..

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I would open it up and look at the connections to see if any can be replaced. Look for any darkened or burned looking connectors related to the display panel. Hopefully you won't need to replace the display panel circuit board but if you do. I would look for the cheapest replacement starting with ebay parts, and search engines for that specific part. Sometimes there are microwaves of different models with the same parts too!

You will need to replace the control board. Here is that part to order.

If it uses a lamp that is possible---sets with a lamp will often dim down just before the lamp fails in a total way.

Press the Setup button ( ).

1. Press the Right arrow button ( ) until Tools displays on the control panel, and then press OK .
2. Press the Right arrow button ( ) until Fax Log displays on the control panel, and then press OK .
3. Press the Right arrow button ( ) until Clear Fax Log displays on the control panel, and then press OK . A message that says the fax memory is clear displays on the control panel.
4. Press the Cancel button ( ) to exit.

Hope this helps

In case you haven't solved this yet:

1. Press the Setup button ( ).
2. Press the Right arrow button ( > ) until Tools displays on the control panel, and then press OK .
3. Press the Right arrow button ( > ) until Fax Log displays on the control panel, and then press OK .
4. Press the Right arrow button ( > ) until Clear Fax Log displays on the control panel, and then press OK . A message that says the fax memory is clear displays on the control panel.
5. Press the Cancel button ( X ) to exit.

You can go to this web page also:
Fax Log Full Message
Thanks for using Fixya.

We can replace the VFD tube for you. The total cost for this repair is $71.06. For more information, our site is linked at our listing here on FixYa: http://tinyurl.com/yzjozk