The pellet, the copper part opposite the spring with the temperature on it, always goes "into" the engine. The spring end should face out.
The bulb end goes toward the engine.
Follow the top hose from the radiator. Where this hose connects to the engine will be a housing connected by two bolts. Drain the coolant remove the hose from the housing end and remove these bolts. The thermostat is under this housing. You will probably have to move the air cleaner tube out of your way to get access to it. Its not a picture, but I hope it helps you.
Not without a specialized scan tool.
There is no way to change gear ratios.
The easiest way is to unhook the battery for a minute.
There is no way to 'test' a timing belt. On your 1993, Mercury suggests it be replaced every 64K miles.
You can't "clear" it unless you fix the problem that is causing the light to be on in the first place. That's the way all airbag systems work.
If the sliding door on the 1999 Mercury Villager will not stay open, check for something that is not allowing the latching mechanism to lock into place. Make sure that the door is all the way back so that it can lock in.
There is no way to do this. There are child-proof locks on the sliding doors.
The same way you do on any other vehicle. Drain the oil, remove the filter, replace, refill.
There has been some talk that the Mercury Villager doesnt have a great cooling system, sometimes a bigger radiator replacement might help.. but either way you have to keep the coolant, timming, etc in perfect state. The reason why it goes from normal to hot then back to normal is because that is usually when the temperature rises hi enouth for the thermostat to open up and lets the cooling back in circulation thru the whole system...
up Spring towards the engine.