What am I missing?

Sounds like the problem is following the guy's coax. A consistent SWR over 3:1 is very often an open circuit in (usually) the center conductor. You've already ruled out his antenna by substituting your own. A great test would be to just ohm out the coax's two conductors from end to end, and make sure the center isn't shorted to the shield. That would be the first thing I'd check, and if were OK to DC resistance, I'd hang a 50 ohm dummy load on the antenna end. If there's *still* the same horrid SWR, I'll figure on corrosion or most of the strands in one of the coax conductors broken, leaving only the tiniest path for the ohmmeter to show continuity, but not solid enough for RF.
HTH,
73

 

What you are missing is the possibility of an antenna ground plane problem. Grounding deals with electricity and bonding deals with interconnecting the large metal components of the truck into one large antenna ground plane. Dead give away is the high SWR reading. I hope that driver has not already damaged his radio by using it with such a high SWR reading.
. This topic - antenna ground plane - has been addressed extensively on this forum.
. Have you ever wondered why some guys have an antenna mounted on their trucks that is pointing downward? That is an alternative solution to the faulty antenna ground plane problem. Now you know.