They sold me the rebar, but my friend said that it was too big for the car and I had to return it. Nothing I could do, it wasn't my car. I had a terrible migraine and was having a terrible day. We still had to get groceries. I didn't get home until after 2am and still had to put the groceries away. It is lots of fun, buying groceries when you are seeing double and throwing up. I don't drive, so I am grateful that anyone is willing to drive me anywhere. I didn't want to force the issue when we were in a hurry and in bad moods. He is going to look around for other sizes of rebar or alternate supports for me. I showed him the trellis that needed support. Eventually, I will fix it.
Practically speaking, one could protect the car with old blankets or bubble wrap and tie a red flag on the part of the rebar that extended beyond the back window. It would be perfectly safe and legal to transport it on the short drive from the store to my home. As I said, it is not my car. Eight foot rebar would be sufficient to support the trellis and fit in the car. That would solve the problem.