Depends on which country, and of course which model. For example, the American truck (2½ ton 6×6 Truck CCKW-353) from Rubicon Models can be assembled with or without the MG.
Now, contrary to popular Bolt Action army lists, most trucks would not have an MGs, so it is not surprising that models do not include these bits as an option. That being said, there are some wartime photographs showing an occasional truck armed with an MG. However, in most if not all of these cases, the MGs seemed to have been mostly field modifications. So to make a long story short, take an MG from your bits box and add it to a truck of your choice.
What I tend to do is not make the MG permanent. For example, with my German truck, I modified an extra MG gunner (from a German half-track) so that it looks like he is firing from the truck. The great thing here is that I can use this same MG figure on any of my Opel Blitz or Maultier trucks.
Also, have a look at this thread:
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