Catch a ride to the GPS destination and smash through the gate to get into the little compound. There's a security guard on the scene, but he's armed with nothing more than a pistol. Run him over.
Get into the massive crane on the water's edge and acclimate yourself to its controls. You need to position the hook directly above the arrows on the crates, turning the arrows green. At that point, you can lower the hook to grab a crate and move it to the loading zone in the main lot. Once you've moved a crate, you get out of the crane and have to inspect the crate's contents. Bash open the lock—and then hope you don't unearth something bad.
Sometimes you'll find nothing inside the crate, sometimes you'll find people that try to beat you down. Be prepared. Eventually, after checking a number of crates, you'll find one that's got Mel's van. Get inside and drive it to the GPS target. You'll get bothered by a few vans with machineguns, but they're not much threat. Just ignore them and drive to the back alley garage. Mission complete.