First test the output side of your breaker using the multimeter to see if it is hot (Has power 12V), Also make sure your multimeter is on DC Volts. If you have power then you should be able to tie into that line somewhere. LED lights have to be wired up correctly, they are not like halogen bulbs that can run on reverse voltage. Test your leds on a battery that works to ensure the LEDS work. Also, leds are very picky with voltage changes, only use 12 volts. If you have a 24 volt system for your trolling motor do not connect your leds here. They will fry is half a second.
1. So test your breaker to see if it works.
2. Test your LED separably to see if it works.
3. Connect your led inline with your nav system and see if it trips the breaker then.
4. If it does you will have to use a bigger fuse as you are using more current then your fuse will allow. This shouldnt be the case as leds typically use very low current.