My Mercury Mountaineer 2002 died and won't start

Have you tested for a failed starter or it's solenoid ,
after it stops & you have nothing ?

Could be your loosing throttle position, if a sensor or
ignition coil is bad,that will shut it down

Check for OBD Codes

This is probably a seat sensor but it could be the pretensioners or the air bag. When the vehicle is started, there is a sensor test that is run on the SRS (air bag) and if it finds something wrong, the light comes on. Given that this is a life safety device and there are hazardous parts in it, you should take it to a dealer to have it worked on.

check fuse, check for fire, if no fire when turning over probably busted timing belt/chain, if fire then check for fuel

So are you saying the warning lights come on, but the engine will not crank ?

Is the battery good in it?

Check the fuel pressure with a gauge. The pump may be going bad

I think you should go for manual first after that it will be easy for u to solve your problem. I had visited this site past 2 months before for my vehicle and find it quite helpful may be you will also get something useful here
Mercury Mountaineer 2002 2005 Factory Service SHop repair manual Download

You have a faulty Brake Light Switch, and it is stuck on.

Replace your brakelight switch. Until you replace it, you could pull the breaker out of the fuse panel in order to keep your battery from dying.

The power distribution box is
located in the engine compartment.
The power distribution box contains
high-current fuses that protect your
vehicle’s main electrical systems
from overloads.
The starter relay is in position #56 of that box.