I think this is a bit of trick question. The factory optional radios for trucks as an "Accessory" is one thing, but the dealers did offer aftermarket stuff in the 20s, whether applied on a truck or a car.
These were generally big honkin' units with a separate control pod and then a large cabinet for the tube radio and controls, and then yet another second cabinet for the speaker. They were bolted up under the dash wherever they could fit.
I have to laugh because I'm sure many of use remember using crystal radios as a kid. You'd hook it up to grandma's steel chair on the porch and PRESTO! you are listening to radio free Europe. It's funny the evolution of radios over the years. To think that we now have 10 speaker surround sound (XM) satellite radios now available from the factory is pretty amazing.
