There is normally not an antenna cable pre-routed. SMB would primarily use a K-40 CB antenna which is a very nice unit. It will quick release from it's base so that you don't always have to have a 60" whip sticking up. It can be a little tricky installing it since it mounts centered side-to-side and just forward of the penthouse on the van roof. It needs to be mounted on metal to work properly. Installation varies with RB & EB, and whether or not there is a fold-down monitor in the cab ceiling. Stay away from the "through the glass" antenna's as they do not perform very well, at least they didn't in the past.
You will need to drill through the roof with a hole saw and be able to tighten an interior nut, so access can be an issue once the ceiling is already installed. If you have a fold-down monitor that can be removed, the job will be much easier. The dome light is usually mounted so far back on an RB, that removing it to try and reach the nut is a futile effort. This is possible on an EB though.