Alexandria Bavarian Service and RRT have been unable to determine why all of my instrument cluster lights stopped working simultaneously.

The cluster lights will work intermittently, but go out after about 30 seconds. Sometimes I can turn off the headlights, then turn them back on and the cluster lights SOMETIMES are back. They go out immediately when I use the AM switch next to the shifter (see #2 below.)

Here is what has been tried so far:
1. All bulbs replaced
2. Headlight switch okay.
3. AM switch next to shifter replaced. Grant at Alexandria Bavarian discovered they went out when the AM switch was thrown. They worked briefly after the new switch was installed, but are out again.
4. RRT thought it could be the Dimmer switch, it checked and is okay.
5. After market radio dimmer switch connection checked.

Any ideas or a suggestions for shop that specializes in electrical issues?