Just because it's mounted on a fiberglass roof doesn't necessarily mean it doesn't have a ground plane. The MT Cat has a good sized chink of foil bonded to the underside of the cab roof. I would expect all tractors of the same vintage to be similarly equipped. I think there are only a couple different cab manufacturers; regardless of the color tractor on which they're mounted and painted to match. Not saying that all cabs are identical but if the same guys are making them and they can soak the manufacturers an extra few bucks for a 'ready to go' antenna mounting place, they'll do it for all of their products.

Running a 50 Watt VHF radio on the factory supplied place for the antenna and the ready installed co-ax to the radio, and its working OK here.