When the Schaefer and Strohminger Ford, Chrysler and GM dealership closed last weekend, it was the second car dealership to leave Cambridge in recent weeks. Preston Automotive Group recently converted its Buick, Pontiac and GM dealership into a used car dealership called CarNet of Cambridge, a move the company said it made to capitalize on the growing market for used cars. Schaefer and Strohminger operated for more than 20 years at the U.S. Route 50 and Aireys Spur Road location. Baltimore-based Schaefer and Strohminger owns five other dealerships in Maryland, none of which are closing, said owner Dave Strohminger. About 25 employees are losing their jobs at Schaefer and Strohminger, but all the staff members of the former Preston Buick, Pontiac and GM dealership have retained their positions with CarNet, and the dealership has even hired a few new employees.