Prerequisites
Brief
Donald needs you to accompany the gentleman from 
Liberator to a meetup in Shoreside Vale. Along the way, the Colombians will try to interfere.
Walkthrough
Bring a strong vehicle like an Enforcer or Patriot. Follow the Securicar until a Cartel Cruiser emerges and do your best to take it out. The target car has a damage meter that must be monitored. Between the start and the tunnel you'll be accosted by a few of these. The Securicar will head into the tunnel near the stadium to follow the undeground route to Shoreside Vale. Don't dawdle with the Cruisers because the Securicar can rack up damage quickly in the congested tunnels, try to take them out fast.
When you approach the Shoreside Vale tunnel exit, try to get a bit ahead of the Securicar. There will be gunmen on foot scattered around the exit, and another Cruiser or two will appear in Pike Creek for the final stretch. You won't fail the mission merely for falling behind the Securicar so if your ride is in trouble you can take a moment to switch to a new one, but the Cartel will usually (not always) destroy the target car in your absence if you leave it alone too long.
Reward
$40,000
Decoy