found this on google, I do not own an etrex so not sure if they are all the same, presumably their sockets will be standard as the data cables appear to be.
Your radio will have data in and gnd. So you connect pin 2 data out to your radio data in, and gnd to gnd. This should work. There may be some fiddling with gnd.
You then need to set up your NMEA. generally 0183 at 4800buad, but this should be in your radio manual. On Garmins, if you have never used data out before it will be defaulted to off. You will need to turn on the data through the set up menus on the GPS.
The remaining connection on the Etrex of course is + input however beware it is not for 12 volts but rather 2.5 volts so to run on shiops batteries you will need a regulator 12 to 2.5 volts. PM me if you need to know how to make one cos I don't know if you can buy one. Etrex is great but my eyesight is not good enough to see the numbers at any distance especially when wearing distance corrected sunglasses. Wish there was a software option to give bigger time and speed read out. regards olewill