I suggest two tests:
1. disconnect the GPS antenna connection from the VHF radio. Connect it only to the Navnet device. See if the Navnet device gets a lock/fix on satellites and gets a GPS position.
2. then disconnect the GPS antenna connection from the Navnet device. Connect it back up to *only* the VHF radio. See if the VHF radio can use it.
There are four possible outcomes:
A. that neither 1 nor 2 work.
B. that 1 works and 2 doesn't.
C. that 1 doesn't work, and 2 does.
D. that both work! when they are not connected up together both at once.
A: means that the antenna is at fault, period. (Or maybe the wiring, somehow. But you've checked it.)
B: means the antenna, and the Navnet, are OK, but either the VHF hookup has something wrong with it, or the VHF radio's tuner is no good, or that the VHF radio's tuner loads down or grounds out or otherwise inhibits the GPS antenna's function.
C: means the same only for the Navnet being defective, not the VHF radio.
D: means... they can't both work (off this same antenna) at once, but all components are OK otherwise.
(Also B might mean the antenna is somehow incompatible (rather than 'broken') for the VHF radio, and C might mean the antenna is somehow incompatible (not broken) with the Navnet. But if the same antenna has worked before with either or both, then it is compatible with them most likely, even now.)