I like
This chart - just assume you have 60C copper wire
If you cant tell what size wire you have (if its not printed on the outside), just go with a 10A fuse to be safe - that'll power most anything you'd plug into a CL. Your GPS probably only requires 1 or 2A to run/charge.
Please tell me that the fuse is on the battery side of the firewall...a 12v car battery can make a LOT of heat if you short it the right way.
-John