Get on the bike with Cesar. Drive the bike across Garver Bridge and get onto the highway. You'll see a petrol tanker which is driving along happily, careening through the other cars. You need to drive to the left side of this tanker and hold in position next to the door. Cesar will then jump from the bike onto the truck and jack it. He'll stop the truck a short way down the road and tell you to get in.
So, get in the truck, and drive the it all the way to the end of the highway, as it is best to turn around when you reach the intersection leading to Foster Valley since you'll need a wide turning area because you can't reverse very well at all with a gas tank attached to the back. If ever the tanker trailer gets unattached, just carefully attach it again to the back of the truck. Follow the road from the intersection and head to the garage where you'll find a red circle to park in.