What about the development work done in the early 90's by GM and the DOE as part of the Clinton era "Partnership for Next Generation Vehicles" (PNGV):
Quote from the book "Energy Research at DOE: Was It Worth It? Energy Efficiency and Fossil Energy Research 1978 to 2000" pg 151:
"From 1993 until 1998, General Motors teamed with Stirling Thermal Motors (STM) to develop and demonstrate a Stirling engine for hybrid vehicles as part of the PNGV initiative. The engine was designed to drive a generator in a series hybrid configuration. Six engines were eventually built by STM, and three were delivered to General Motors for testing. By the end of the program, the Stirling hybrid propulsion system was integrated into a 1995 Chevrolet Lumina. The Stirling hybrid vehicle failed to meet several key requirements. Specific shortcomings included lower-than-expected thermal efficiency, high heat rejection requirements, poor specific power, and excessive hydrogen leakage. The engine did meet its emission target, demonstrating half the ultralow-emission-vehicle (ULEV) standard. There are no plans for further development of the Stirling hybrid concept with GM or any other auto manufacturer. STM is working to commercialize a small Stirling-powered generator for commercial use."
And this fact sheet from the DOE Office of Advanced Automotive Technologies:
http://www.p2pays.org/ref/16/15287.pdf
Looks like they did have a prototype but it turned out to not be economically feasable at the time.