Your expectations are not too great.
Referencing page 5 of your Garmin
installation manual, your machine has
NMEA 0183 connections (combined with the
power pigtail).
NMEA 0183 uses RS422 protocol, which is compatible with
RS232 for these purposes, which is the standard 9-pin
serial port or DP-9 on a computer.
If you don't have a
serial port, you can use USB with a serial to usb converter, sometimes called a serial emulator. They are like $10.
To transmit/receive
NMEA 0183 to a computer, your will use Garmin wire 3 (Receive/Transmit (B) -), wire 4 (Transmit (A) +), and wire 5 (Receive (A) +) per manual page 5. Tie wire 3 into wire 2, the ground.
The DP-9's serial pins of interest are 2 (Receive), 3 (Transmit), and 5 (ground).
Google "RS232 pinout" for graphics (image search).
So, going from Garmin to DB-9:
3 goes to 5.
4 goes to 2.
5 goes to 3.
Again, make sure Garmin wire 3 is tied to ground (Garmin wire 2).
Most programs designed to digest
NMEA data will have instructions on how to allow the program to receive the data. It usually entails a setting within the program to allow
communications to COM1 (or whatever COM the serial port is tagged as).
OpenCPN for instance will receive/transmit NMJEA data to from a
chartplotter suing this method. I have also had it working with SeaClear.
If you call Garmin, the above is exactly what they will tell you, as some of their older units came with more detailed instructions, as
RS232 interface was more common back in the day.
In case you don't have some old computer serial cord laying around, just go to your local
electronics parts store, and buy a male DB-9 solder connector, and a few feet of shielded 3-conductor cable. (I used telephone cable once and it worked fine, and sometimes an old length of traducer cable works.) If the cable has more than 3 conductors, just don't use all of them, and pick your 3 favorite colors.