So, last weekend I fixed the dashboard lights on my Series 3. First, there are no individual little bulbs for the lights, it's all fiber optic. Now, after eading the forums, I came to the conclusion that the problem was the rheostat. A.K.A, the dimmer switch.
Here's what to do:

1) Pull of the knobs on the heat and fan switches
2) On the switches there are threaded washers that hold the face plate on. Take a screw driver or pair of needle nose pliers and turn them counter clockwise and remove them and pull the face plate off. Note: there are fiber optics back there going to the heat control slider, they look like black wires, but they are not.
3) Get a flashlight, put your feet on the ground, lay down on the seat and put your head on the console...just do it.
4) The upper face plate is held on with two screws (bottom) and two tabs (top). Shine the flash light in where you removed the face plate and you will see the two black screws that hold the upper face plate for the interior lights / trip computer. Take them out, pull on the bottom and drop the face plate down. You now have access to the rheostat.
5) There are three wires going to two tabs on the switch. the upper two of the wires are connected into one spade connector. The switch has three tabs, one on the bottom, two on the top. The two tabs on the top are connected to each other internally, but only one tab is used (on mine)
6) Take the wire off the bottom of the switch, and slide it onto the unused tab at the top of the switch. This effectively connects all the wires together, thereby bypassing the failed switch. TA-DAH! restored dash lights.

The dash lights on these cars are dim, so you wont see them untill its dark enough.

Assembly is the reverse. Total time to repair and reassemble? 30 minutes.

 

Good work! The rheostat bypass is a good trick. I did it on both my Ser III and my XJS.

To clariify, though, fibre optics are used for only some of the panel illumination such as ignition switch, headlight switch, and climate control panel.

The gauges and the four push-push switches use individual bulbs

Cheers
DD

 

AH-HA! So the fuse I replaced for the sake of replacing the fuse inside the console must be for the gauge lights! I fixed my windows/sunroof this weekend and had that cover off as well. Saw the inline fuse and replaced it "since I was down there", not knowing what it went to!

 

No, the fuses in the console are for the radio and power seat.

Cheers
DD

 

Does this procedure brighten the dash lights any? I find them to be a little dim even with the rheostat on full blast. Is it worth doing if the rheostat is functional?

 

Oliver- The procedure basically takes the power and makes it a direct connection. This is the first I've seen my dash lights since I bought the car three months ago (been using a small LED light from rite aid as a substitute until I had time to look into repair) They are a little dim, but that's ok. I assume it's like the windshirld wipers being slow...ya just gotta live with it. If your rheostat is functional, I wouldn't bother.

Doug- I was thinking backwards when I responded to your post, over exited about dashlights I guess.(duh...bypassed the rheostat...) The fiber optics weren't working prior to the bypass si I imagine they sprung back to life when I removed, cleaned and retightend every ground wire I saw down in the console....

 

On both my older Jags bypassing the rheostat made the dash lights brighter, yes.

Cheers
DD