1995 C280...the constant power behind your radio probably has a color code red with a blue line...
You can use a test light to check all wires for power.
You will have a brown ground wire, a pink/red switched power, a gray/blue illumination, a green/white speed-pulse, and some other wires.
Never connect a wire directly to the battery (especially NOT without using a fuse). The C class has some wires connected to the positive side of the battery and these wires run directly to the additional fuse box in the trunk. Lift this fuse box out of its socket (no tools needed), turn it 180 degrees round and you will see a lot of free pins at the bottom to which you can connect any permanent powered wire. Take a pin that leads to a spare fuse position and use that for any additives in your car and run the wire through the existing cable trays to the front.
It is possible to unscrew the lid over the front fuse box as well (after you have disconnected the battery), take out the parts that hold the fuses and look at the bottom side. It is possible to connect additional wires here as well...
with a minor 600+k