First off, when using DyMESH we strongly advise that you use a smaller integration timestep and output time interval than the normal defaults for an event. The recommended values are conveniently found in the DyMESH dialog, under Options, DyMESH. When you check the box to Use DyMESH, go ahead and check Use DyMESH Time Intervals as well.
Our other advice for your case is that you should always tessellate Generic vehicles. Tessellation reduces the polygon size (polygons on Generic vehicles are huge), thereby improving the interaction between vehicle meshes, and also providing for better definition of the crush deformation from the collision.