Harness with seat belt attachment-I swear by them as they can be used for walking on the leash then you change the attachment and hook them into the car, easy transition and safe the whole time you can find them everywhere (here's the link to the ones I have wish I had found them here as they are really cheap on eBay)
Dog Car Safety Restraint Seat Belt Harness 7-15" Chest - eBay (item 350373860862 end time Jan-09-11 18:01:03 PST)
...personally I don't think anyone should ever have their dog unsecured (safely restrained) in their vehicle period, for the safety of their dog, themselves or others-think what would happen to your dog during a minor fender bender with him being unrestrained also...
Dog and children need to be forcibly protected by us for the same reasons...I'm so sorry you had to go through this, but better to go through it have it all turn out okay and then learn from it before it happens again!
As far as running being an issue use a leash on his harness always with the exception of fully fenced or secure areas when you can work on training-
I will never be able to unleash my Schnorkie Scoobers, he catches a scent and before he can think....he's bolted....gone....running....following his nose...
we've spent two years on it now, he'll always be leashed when in an unsecured area-he comes on command now, but I can't risk his scent response and immediate bolting before he thinks and comes back to me.