According to the Auto Cheat Sheet, car salespeople are paid a percentage of the dealer's profit, typically around 25 percent for new cars though this can vary greatly from business to business.
Most car dealers offer commission-paid employees what is known as a "draw," which guarantees the employee a set amount of pay even if they don't earn any commissions. Draws function as a type of loan from the employer in that the employee is expected to pay back the draw from future commission earnings. Commission and draw structure varies greatly between dealerships. Whether it is a new- or used-car dealer can also play a large role in the pay structure.