Where is thermostat?

It is located under the thermostat housing at the rear of the engine which is on the driver's side of the vehicle. Follow the top radiator hose back to the engine, and there is where the t-stat housing is located. If you have a 3.0 engine, it is held on with three 10mm bolts. If it is a 3.8 liter, it is held with two 10mm bolts.If you have the 24 valve engine it is located at the lower radiator hose

Read this guide, not vehicle specific but it will give you an idea of how to change it

https://www.2carpros.com/articles/replace-thermostat

Vehicle specific instructions/diagrams are attache below for the 3.0 and the 3.0 24v engines

Try below

On the OHV Engine (Vulcan Engine) follow top radiator hose from radiator to engine. Disconnect top radiator hose, remove the three bolts, pull off thermostat housing and remove thermostat.

OHC Engine, drain radiator until the coolant level is below the lower coolant level on the engine. Raise vehicle support it with jack stands. Disconnnect the lower radiator hose from the coolant inlet housing, remove the two inlet housing bolts and the housing. Then remove the thermostat and the O ring from the thermostat housing.

Hello,

There are two different placed depending on which engine you have sop here is a guide to show you what you are in for when doing the job.

https://www.2carpros.com/articles/replace-thermostat

Here are the thermostat locations

Check out the diagrams (Below)

Please let us know if you need anything else to get the problem fixed.

Cheers

Upper radiator hose
[engine side ]
two bolts holding down the housing

Replace the thermostat-start here

Drain the engine cooling system so that the engine coolant level is below the water thermostat.

Disconnect the upper radiator hose from the water hose connection.
Remove the three water inlet connection retaining bolts. Remove the water hose connection.
Remove the gasket and water thermostat from the water hose connection. Do not reuse gasket

3.0L V6 - VINS [U,1 & 2] -1999 Mercury Sable LS
Page 1 of 1
THERMOSTAT

Warning: never remove the pressure relief cap under any conditions while the engine is operating. Failure to follow these instructions could result in damage to the cooling system or engine and/or personal injury. To avoid having scalding hot coolant or steam blow out of the cooling system or degas bottle, use extreme care when removing the pressure relief cap from a hot cooling system or degas bottle. Wait until the engine has cooled, then wrap a thick cloth around the pressure relief cap and turn it slowly until pressure begins to release. Step back while the pressure is released from the cooling system. When certain all pressure has been released, press down on the pressure cap (still with a cloth), turn and remove pressure relief cap.

Removal
1.Drain the engine cooling system so that the engine coolant level is below the water thermostat.
2.Raise and support vehicle on hoist.
3.Disconnect the lower radiator hose from the water inlet connection.
4.Remove the two water inlet connection retaining bolts. Remove the water inlet connection.
5.Remove the "O" ring seal and water thermostat from the water thermostat housing. Inspect "O" ring for damage and replace if necessary. 6.If required, remove water thermostat housing retaining bolts, water thermostat housing and "O" ring from lower radiator hose tube. Inspect "O" ring for damage and replace if necessary.
Installation
1.Install the water thermostat, "O" ring, water thermostat housing (if removed) as shown.
2.Install and alternately tighten the retaining bolts. Tighten bolts to specification.
3.Connect the lower radiator hose to the water inlet connection.
4.Lower vehicle.
5.Fill the cooling system using Premium Engine Coolant (green in color), E2FZ-19549-AA or equivalent meeting Ford specification in a 50/50 mixture. Bleed system .
6.Start the engine and check all hoses and connections for leaks. Allow the engine to warm up so that the water thermostat opens.
7.Stop the engine. Top off degas bottle as required to maintain minimum cold fill level.

Follow the upper rad hose. Pretty staightforward R&R. Check your PMs.

Good luck and laet us know. :)

You need to check if heater hoses hot when engine at operating temp
both heater hoses should be hot if not you need to isolate heater core hoses from engine block bypass hose in-between and flush heater core
if both hot and no heat check blend door and actuator
have to remove radio to check