A group of Chevrolet Camaro cars for sale is pictured at a car dealership in Los Angeles, CaliforniaThe non-revenue workforce, often called “I am the Cavalry,” is asking attendees at this weekend’s Def Con hacking convention in Las Vegas to sign an open letter to “Car CEOs” to ask them to put into effect basic tips to defend cars from cyber attacks. “Now could be the time for the automobile industry and the security group to attach and collaborate.” Automobiles depend on tiny computer systems to control the whole thing type engines and brakes to navigation, air con and windshield wipers. The Cavalry group is scheduled to make a presentation at Def Con on Saturday about efforts to improve auto safety. They will not reveal any particular problems that may embarrass carmakers, said Josh Corman, a security trade professional who co-founded the group a 12 months in the past.     That sensitivity contrasts with much of the hacking analysis offered at the moment at Def Con, which attracts more than 10,000 attendees.