For pulling out a nice Service Menu out of the Radio, do the following:

• Switch the radio on
• Push "TP" for at least 10 seconds

A "Service Menu" will appear. You will be informed about

• Serial Number
• SW Version (mine is from 15/94)
• Seek Level ("I" for reception of strong radio transmitters, "II" for reception of all radio stations)
• GAL (Geschwindigkeits Abhaengige Lautstaerkeregelung - speed dependent volume control). Set it to 1 for small volume increase when driving faster, and set it to 4 for even more volume when driving faster. Mine is set to "1" and still too much).
• DSP (If your car is equipped with the HiFi Professional DSP, you will find an "1" here. Otherwise a "0" (like mine).
• FM (some information about the radio station you are currently listening to. Includes also quality of reception. )

To get back from this menu, simply switch off the radio.

Source: тынц