So, I cut the ends of my cable and reattached the field connectors. I used my multi-meter to verify it was wired correctly. Same problem. My cable doesn't work but the pre-fabbed cable does.
Than I did something interesting. I put the pre-fab and mine together with the pre-fab one at the Garmin
740 end and it worked. So, the problem is that the field connector wasn't making a good connection at the Garmin
end. I attached the field connector to the Garmin again using wrenches to get it all the way down and very tight and it worked. I had been very careful the previous times I connected to it so it wasn't like I over looked this before.
I blame the low quality plastic connector on the back of the Garmin. Everything works now. I just hope I don't have to touch that connection again for a long time. It might not stand up to being removed and put back on tight enough. No such problems with the metal connectors on the iCOM