If you are providing custom training materials for clients, I've found it's much easier to just take photos of the trucks on site. I've contacted manufacturers in the past for use of images and it's kind of hit-and-miss as to whether you can ever find the right person to talk to and if they are willing to give you permission. If you have access to the vehicles, just take the photos yourself, it saves a lot of time.
If you absolutely want to use images from a manufacturer, I prefer to try to contact them via email (it saves time rather than holding on the phone, and their response provides a written record of the permission approval if granted). Send them an email with a clear and detailed description of which images you intend to use and exactly how you intend to use them.