I booked a res using 5299328 (listed as Amex Family Pass or Carlson Travel) and the agent asked me if I worked for GMAC. Apparently this rate is limited to GMAC employees, and requires a GM car to be provided. I told her I'd gotten the rate from a friend but was not an employee. Happily, after much discussion, she and her supervisor agreed that they could give me a comparable rate, but I'll definitely not be borrowing this rate again!