If you explain in more detail what you are trying to accomplish?
Regardless, the Garmin wiring
diagram is crap. The yellow wire gets connected to DC- when there is an alarm
(you have to have an alarm configured in the Garmin, when that alarm is active the yellow wire is DC-, otherwise it goes nowhere). So, connect a DC+ (from the same source as the Garmin power supply) to the coil of your relay. Connect the Garmin yellow alarm wire to the other terminal of your relay coil. When the Garmin alarms the yellow wire will get connected to DC- inside the Garmin and your relay coil will be energized. Whatever happens after that depends on what you have wired to the relay contacts.