Try to head them off. If I know the cause I'll tell him why I think he's upset. "you want your toy? Are you angry because mummy took it away?" He will usually say "yes" then I explain why I took it away (again) and find something else to do together for a few minutes or longer if I can.

Mid tantrum I do something loud our silly to get his attention then repeat above.

It's prettty rare for this to not work. If he's still going I just stay nearby and tell him I'm here for a cuddle if he wants one and leave him to it.

ETA- could the car seat be an independence thing? Ds has just decided he has to do EVERYTHING himself.