Talk to Hank who is standing by a car downstairs in the Lifeguard Station to start the quest. You need to find 4 car parts from car wrecks. The car parts should get marked on your mini-map when you are near them if you have started the quest.
There are four broken down cars on the road between the Lifeguard Station and the Lighthouse.
After you get the four parts return them to Hank.