The reason the mesh needed to be rotated was because its heading was off by 90 degrees in yaw. But, that was specific to that particular mesh. In general, you'll just need to ensure your vehicle's heading is at yaw = 0 degrees. The best way to do this, as was demonstrated in this blog post, is to first make a "test vehicle" from the left side control panel to confirm the proper orientation in your workspace. All vehicles from the Virtual CRASH database import at the correct default orientation. Match your third-party mesh's orientation to match the heading of the Virtual CRASH model. Once you do this you are guaranteed to have agreement when you import your vcm file.

In the figure above, we see the imported BRZ model is oriented along the Earth frame y-direction (yaw = 90 degrees). So, if one tries to use it as a mesh for a pre-existing vehicle, the mesh will be imported with heading at yaw = 90 degrees. The BRZ mesh first needs to be rotated such that its heading is pointing along the Earth frame x-axis (yaw = 0 degrees). The quickest way to ensure you know what direction the Earth frame x-axis is pointing along, is to simply bring in a "test" vehicle model from the database, which already will already have its heading at yaw = 0 degrees. Using this to sample the Earth frame x-axis direction, you can then easily correct your third-party mesh's orientation.