I can't help with the Garmin pins. However, use the settings found at
http://www.aprs-is.net/dprs.aspx (you will need to calculate your C1 message on the dprscalc.aspx page, too).
The manual is a bit misleading. You do not (cannot) set My Position. You are checking to see that you are receiving lat/lon from the GPS.
Also, the pins are a bit counter-intuitive. RS232 IN means
into the
attached device and RS232 OUT (GPS) is output
from the
attached device. It has been a while since I have made a cable for the 800 but that is my recollection.
Hope this helps.
73,
Pete AE5PL