Instrument Lighting A3

Hi,

My girlfriend has a 1996 A3, and I recently replaced the original Audi radio with a slightly better CD radio. In the process, I purchased an adapter that allows me to swap the ignition and power connections. Furthermore, there is also a connection for instrument lighting, which I have taken over 1:1.
The lighting on the new radio was quite dim. Since we received the radio for free, we initially thought it might be a problem with the lighting itself, but yesterday I checked it and it was fine (they're LEDs, which rarely break). Then I measured the lighting voltage and was surprised to find that even with the dimmer set to MAX, it only showed around 4V? The Audi is also a bit different in how it's shifted, because there, the speedometer is already illuminated when the ignition is turned on.

Does the Audi have a 4/5V lighting system (or essentially just a control circuit)?
So far, I thought that the instrument lighting was always dim 12V. Or is the dimmer perhaps defective (I don't think so, since the other lighting is very bright.)