Dear Derwood60,
I also had a head injury in a car accident and have just been diagnosed with pots. Believe me, you are NOT alone. POTS can develop after of injury or trauma, (among other things)and a car accident is both.
The only connection I have found between POTS and head injury is the damaging of the medulla. The medulla is responsible for your autoimmune system which is failing us both right now. I'm not sure if the damage has to be significant enough to show up on an MRI or not.. my MRI's came back normal--but I still have POTS. so who knows.
Eating more salt has been advised, but make sure to increase your fluid intake as well. This increases blood volume.
Best of luck, hope this helps.