Radio question for you all,

My 97 burb radio was pulling an amp from my batteryís with the ignition off. The parasitic draw dropped to an acceptable 39mA, when I pulled the 10A radio fuse.
I was slow to look into it, re-charged my 6yr old batteries a few times, till they didnít seem to hold a charge anymore, then I had to pay the piper. To protect the new batteries I threw in a relay between the two legs of the 10A radio fuse socket, so itís only connected when the ignition is on. Yes I put an inline fuse holder with 10A fuse.

Hereís the thing, I noticed the radio channel presets stay intact the next time I start the truck.

Iím curious, does the radio have a battery, or a capacitor the memory draws from, or does the radio use a secondary 12v source for memory that doesnít go through the radio fuse?
I re-checked my relay wiring, pulled the wire plugged into the dry leg of the 10A radio socket, meter reads zero volts with ignition off, 12+ volts with ignition on, so the radio is definitely being disconnected from the battery at that fuse juncture with ignition off.

This has solved my battery draw problem, since Iím no longer seeing an unacceptable parasitic draw (anything above 50mA).
I do loose the clock, as it always defaults to 1:00, but Iím curious about not losing the radio channel pre-sets.
Does anyone out there know why?