Mission 28 - Tail Bagman to Union Stash
One of those missions where the greatest enemy is the mission interface itself.
Cruise around the zone until you see one of the red cars pass by (they'll also show up in your minimap). You must be in a vehicle, and on the road, to activate the next step.
Having the car in your sights is not enough to trigger the next mission phase- you have pull in close enough that the 'TAILING' alert appears at the top of your HUD. If you don't make this initial close contact, the target car will simply loop forever.
Once you've engaged the tailing phase, you can drop back in distance. The key here is to drive in such a way that no one notices you- don't break the law, don't evoke the honks of other drivers, and don't commit vehicular manslaughter. At least, not much. A warning bar at the top the screen will tell you how close you are to tipping off the mark.
You may actually find this easier in a slower vehicle, so as not to risk overdriving (I used the truck from the warehouse robbery). If you complete the mission, the driver will exit the car with a 'KILL' icon over his head, looking exactly as if you'd failed mission, only this time he'll walk inside a nearby building instead of opening fire. Take position against the outer door, and whistle-kill the lot of them, before heading inside to claim your spoils.
See all the details here.