They are attach in a similar way to a prop but use a section in the truck files called flexbodies, which makes the mesh flex as if it is a spring. This is what that line looks like.
4,39,44, 0.0, 0.0, 0.0, 0, 180, 90, 32CoilRear.mesh
forset 4, 39, 22, 37
the forset line is what defines what nodes of the truck are use to simulate the deformation.