Yah... it's one of those hard phases in a kid.
No telling when it will pass.
Both my kids went through that too.
What I used to do, is, my kids LOVED those helium balloons which my local Safeway has.
And I would tie it to the armrest of the car... but so as not to block my rear view mirror, and not in a place where my kids could get tangled in it.
BUT, it kept them entertained in the car. And you can use it as an incentive. The helium type balloons last a long time. I just kept it in the car, tied down.
At this age, they are getting the desire to be more independent, and yes, they get more "opinionated."
A great book is "Your 1 Year Old" or "Your 2 Year Old" from www.amazon.com
Its a book series on each age-set of a child. Fast easy read.
You could also just give him a head's up about what is going to happen next, ie: "After Mommy pays for my things, we are going back to the car... help Mommy." Or, have him actually put something away in the car for you like a little item.
-Some kids need a LOT of forewarning... before an event or what is coming up. Its about the "transition" from one thing to another. A "transitioning" issue. So you have to prep them on what is coming up, and telling them your "expectations" on it. ie: "in 5 minutes, we are going home..." and then give him an incentive for getting in the car...and then going home, ie: "when we go home, you can play with your truck..." etc.
And then too, he probably does not like being "strapped in" a seat... as most kids. But then once you get in the car, the movement "soothes" them.
Also, running errands/doing appointments is harder if the child is tired. Perhaps timing your errands for after his naps... or before...but BEFORE he gets tired. Its always harder running errands when a child is (1) tired or, (2) hungry.
For me, I run my errands BEFORE my son has his nap time, or after his nap. I go to the stores that opens early or are open 24-hours.
Or for my friend, she just did her errands when her hubby was home. That way, she could do it by herself.
Well, hope the balloon idea helps...
All the best,
Susan