Four Winns typically (at least on both of my FWs) wire the yellow and red wires that are on the stereo harness together, and then to the battery bus.
The red wire is usually the + lead, and the yellow wire is the keep-alive memory power. Most aftermarket stereo systems amazingly generally use the same color code, but it doesn't hurt to check the stereo's owner's manual.
What you want to do is to disconnect the yellow wire, then wire it directly to the battery. But to meet USCG regulations, you have to fuse the yellow wire
at the battery. A one-amp fuse should be sufficient. And maybe an in-line fuse holder such as this Ancor waterproof fuse holder:
You may also want to wire an on-off switch (maybe hidden), so that you can turn the memory off during the winter. Even though it does not take much juice, I always felt better having everything disconnected from the battery in the winter.
This is the switch I have on my current boat. Its a toggle switch, but cannot be flipped unless you pull up on the collar, which unlocks the toggle. This prevents accidentally turning the switch off.