I guess your radio switches off when you stop the engine or take out the
key?
At the back side of the radio, you have this ISO connector.
From the car there are 3 wires for 12V going to this connector.
Black is the minus (min)
Red is 12V+, this one is switched on when you turn the the ignition lock to
ACC.
Yellow, is 12+ permanent direct from the battery, this wire is always 12V+,
also when the engine is stopped and you take out the key from the ignition
lock.

But to be sure, just measure it with a multi-meter (volt-meter), with the
ignition lock
off en in ACC. It is possible that these two wires are turned around.

If you want your radio to be on all the time?, than just connect the 12V+
permanent
from the battery, to the red and yellow wires of the ISO-connector on the
radio.
So the radio will always be on, even when you turn of the engine and take
out the key.
And take the chance, that you forget to shutdown the radio, so your battery
will be
empty :-) And than you can call the ADAC to help you :-)

Good luck, I hope this info will be helpfull for you.