If both devices behave "like hell" or do not show anything at all, then it is most likely due to the stabilizer. Someone who understands electronics will be able to buy the part using the printed illustration and replace it themselves. Otherwise, you need to change the printed circuit board (see "Disassembling the instrument panel").
Checking the voltage stabilizer
Remove the instrument cluster and reconnect the wire.
Turn on the ignition.
Connect a voltmeter between the middle and upper contacts of the stabilizer (if you look at the instrument cluster from the back).
The voltmeter should show 9.75-10.25 V.
If the voltage value is outside these limits, then the voltage stabilizer is faulty. If there is no voltage at all, you should check the power supply.
To do this, connect a voltmeter between the middle and lower contacts of the stabilizer. If it does not show battery voltage, then you need to look for a break in the wiring.
[The original source of the article can be found on the website: AudiManual]