I can understand your apprehensions about your symptoms because you have had a number of tests without reaching at a diagnosis and yes, mainly because it is a very dangerous thing to happen while driving.
Dizziness while driving can be from driving anxiety, claustrophobia of being in car, carbon mono-oxide leakage (get this checked at a mechanics), subluxation of C1-C2 vertebra (a MRI of this spine should be done). Sometimes it is due to fluid in the cochlear canals of the ear. It can also be blocked Eustachian tube.
If the symptoms is due to driving anxiety then you will not feel this symptom if you are in the passenger seat. So, for a few days, go with a friend or relative on the same route that you usually take but with you in the passenger seat and your companion driving. See if you face the same symptoms or not. If it is due to claustrophobia of being in a car, then you will face the symptoms irrespective of whether you are driving or not. In this case, see if driving with car windows down or in a bigger car helps. Also, if it is due to claustrophobia, your companion will not get similar symptoms.
If there is carbon monoxide leakage, then driving with windows down will not cause the symptoms. Also, your mechanic will be able to detect he leak. You will get the symptoms irrespective of whether you are driving or not. And if you get the symptoms in the passenger seat, your companion also most likely to feel the symptoms. Also, if you go to ER immediately, they may be able to detect carbon-monoxide in blood.
To detect subluxation of C1-C2 vertebra you need MRI of neck in specific position. Please discuss with your neurologist. Fluid in cochlear canal of the ear can be tested for by an ENT. If it is blocked Eustachian tube due to allergy to something in the car (may be mold in car AC—get this checked) then doing a valsalva maneuver (take a deep breath and blow while pinching your nostrils shut and closing your mouth) will help clear the secretions in the Eustachian tube that connects the back of your throat with your ear. Doing this maneuver 2-3 times after taking steam gives the best result.
Since I cannot examine you and know other related conditions you may be having, nor is a detailed history possible on net, I have listed the various possibilities that should be looked into. Please consult your treating doctors for this and follow the steps I have mentioned to see if they help sort out your problem. Good Luck and take care!