
I would try giving him his meals in the car - not driving him anywhere, until he is a little more confident that nice things can happen in cars. Then you can sit in the (stationary) car with the engine running, just a couple of minutes at a time, then let him out again. Build up the time (read a book or something - just sit there and ignore him) until he loses some of his fear.
It could take a long time.
I'm sure his reluctance to be in the car will be related to the accident - my dogs and I got hit by a car when we were walking 8 years ago. We all survived relatively unscathed, but the one I have left from that time (she was 15 months old at the time) still trembles when we walk down that particular stretch of road (it took a year to be able to get her to walk past the spot).