My 1993 Lincoln Mark VIII had one. If you run into a Mark VIII at the wreckers, you can get one there. As far as I know, they were installed in all Mark VIIIs, all years (1993-1997), not part of a special package of any sort. I would think that it could also be used as a rear end cooler, if one were to need one.
Along the same lines, they also have air compressors built in, as all cars had air bag suspension on all 4 corners (as opposed to a load leveller suspension, which would only have the air bags in the rear).
Kostas