You need to get into the cab and drive it over to the tanker. Back it up slowly to the tanker to connect the two. Now you have a health bar and have to drive carefully over to the indicated spot. Of course nothing is easy, you have a car chasing you and shooting at you too. You're on steep roads here, so your trick is to "nudge" that car off the cliff so it bursts into flames.

Mission passed
$5,000

Now you can get trucker missions there if you want. The cell phone rings - The Truth calls. "We have a mutual friend". You get a TT symbol now.

There are flowers across the way here. Grand Theft Auto San Andreas Walkthrough