That all gets too involved to answer on this board. Easiest thing to do is get a repair manual from your local parts store.
How do you change the thermostat in 1999 Mercury Villager
Remove radiator cap. Push Cougar out of the way. Push a better car under the radiator cap. Install radiator cap.
when should i change the timing belt on a mercury villager
To change the thermostat on a 2002 Mercury villager, pull the hose away. If the bolts do not come off, wiggle the fitting to expose the thermostat to be able to change it.
There is no change transmission lamp.
If it is a 3.0 then the answer is yes. Change it at 105,000 miles.
I assume it is the same as the ( 1996 ) Mercury Villager : With engine oil filter change : ( 4.2 US. quarts of 5W-30 engine oil )
No, you do not have to remove the engine.
There is no way to change gear ratios.
1999 - 2002.
Recommended interval is 105,000 miles..
According to the 1999 Mercury Villager scheduled maintenance guide : ( 105,000 miles ) It doesn't hurt to do it sooner since the engine is an interference engine