Once all three have been found, focus your attention on the annihilation of service stations. Blast open the door at its base and drive in. Then, just keep your wheels pointed downhill and you'll get a briefcase. Another can sometimes be found at the top of the prize stand. Between the A-frames and the lodge is a small ramp.
Briefcases can be here, or behind the lodge.
Otherwise, scan the icy cliffs for stragglers. Otherwise, look near the crates themselves. Once in awhile, you'll find one floating above the water in the docking bay. The warehouse is another place to find them-take the various doors to transport to the upstairs exits. When you have collected 4 boxes, Objective A will be met.