Elvis's Cadillac station wagon (VIN 6D49R2Q163176) is cream with a black vinyl top, leather interior and has a small gold plaque on the passenger side door engraved with "TCB." It was auctioned by Guernseys on October 7th-8th 1999 at the MGM Grand Hotel in Las Vegas for $34,500. It has four regular doors and the fifth door on the back opens two different ways.
The car actually started its life as a 1972 Cadillac Fleetwood sedan that was shipped from the factory in Detroit to the American Sunroof Company where it was converted in to an Estate Wagon.
http://elviscadillacs.tripod.com/ and http://www.classyauto.com/v/Cadillac+Custom/Elvis+Presley+ Via: http://stipistop.com/
Thanks to Jalopnik, I just learned that while the provenance needs verification, one of five custom 1969 Cadillac wagons used to transport VIPs at the Indianapolis 500 has turned up in Putnam NY via Ebay.
Reportedly, all five of the custom-built Indianapolis 500 cars were eventually sold by a Cadillac dealer in Dallas. Apparently it cost as much as a fully-loaded Corvette Stingray of the time ($6,500) just for the conversion.