Hi matey.
I had this exact problem where it says you have to regroup at the cars.
What I had to do to get past this was redo the whole mission before it would finally let me move on. I know its hassle but it has to be done :\
I first tried replaying the checkpoint but that didn't work so redo the whole mission and that should work. Worked for me so good luck. Shouldnt take you too long.
Ben