attn: 99 and later F-motor owners
 

The thermostat is mounted by the A/C compressor where the lower rad hose meets the block...Not above the tranny bellhousing...Almost made that mistake(Damn repair manuals :mad: !) There is a water neck fitting above the tranny beelhousing, it has a gasket, but the thermostat has a build in rubber seal.
I guess the earlier motors like in the Probe had the thermostat above the tranny.