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.

Alon

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'92 300CE - Sold
2004 C240 - 744 - C7 Wheels - Android Radio
2002 C320 - 816 - Sport Wagon - Sold
2006 E350 Wagon - 368 - Dead
2015 GL350 Bluetec - 197U