I wouldn't let my kid be driven around in a car that's smoked in now. I certainly wouldn't have done it when he was a brand new baby.
I'd probably cop out and tell MIL that the pediatrician told you the baby can't be in the car due to the third hand smoke being bad for LOs' health.
Then I'd tell my DH to grow a pair and that his kid's health is more important than his mother's feelings.