The decimal place problem applied to the JD Brown box displays, but I don't think the 2600 had that problem.

Are you sure the AgExpress cable is supposed to accept GPS on the DB9?  I would start by hooking something else to the NAV2 and verifying that it is outputting a good NEMA message.  If so, that should narrow down the problem to either the cable or comm settings.  Could be that the AgExpress cable is bad.