Without digging _too_ far into the document pile, I think the roof aerials were only used with the very early "number" sets, WS.2, WS.3 and WS.9. I suspect the sets are rather more scarce than the Gin Palaces, these days.
There's a whole supplement to Signal Training Volume III Pamphlet No.25 that deals with the WS.9 in a vehicle.
There's also an Installation Print from March 1942 detailing how you fit a WS19 Mk.II Truck & Ground station to a 15cwt truck, and it has all the necessary parts listed on the diagram.
Chris.