First of all, no need to rush chasing him. You can't catch him anyways. Just open the door and move on.
There aren't many police yet. But always be careful.
When police start to ambush, it will be hard. Use anything to take cover. To survive, it all depends on your skill and teamwork.
If too many police, you have to slowly move and take cover from car to car. It is the only way. But do not seperate, move as a group and try to stick at least two players. By doing this, you can avoid getting shot from behind. The other two should take different route.
This is where you should be.
You have to find fuel to heat up the van. From time to time, you need to get more fuel to make more heat so that the guy will come out from the van. Defending this area is not easy, but you can always find place to cover, heal and reload your ammo.
Finally, he comes out. Now, you have to force him to walk to the destination point.
Don't lose sight of this guy. He will stop walking. You have to force him to walk again.
Beware of snipers here.
When you reached the destination point, wait for the helicopter.
Clear the area.
Now, force him to walk into that circle to end this mission. [Mission Completed]