I know this thread is old, but, it is 2016 and highway cables still seem rare. Rare enough that I may just make my own since I am already equipped to terminate mil-spec connectors. I wanted to share that the jacks & plugs on the VIC-3 (and, as far as I know, the VIC-5) seem to be MIL-DTL-26482. The radio ports are apparently a 10-6 pattern and the highway cables are 10-7. These jacks and plugs are available new from the usual civilian electronics sources although new they are not cheap ($60 ea). I would not be surprised if they turn up for less on a certain popular auction site, as mil-spec connectors are used for a lot of things and find their way into the used / NOS market as components.