just run your own secondary memory wire to the radio. then keep it connected to the battery and disconnect everything else that doesn't need constant power.
They sell a new battery terminal for your positive connection that will retain memory but allow the battery to be disconnected with a simple twist of a knob.
'92 300CE - Sold
2004 C240 - C7 Wheels
2015 ML350 - P1, Pano, Ash poplar wood, Sport Dinamica interior, Running Boards Keyless go.