If locks have also stopped locking automatically, then you will have to change the brake switch located on the brake pedal. Go to a local auto shop and pick one up. It is easy to replace. Just look how the original is mounted, it is not too complicated.
bad switch or possibly stuck switch
back up close to a door or building... press brake pedal... look into mirror to see if lts are lit
If the ((ABS)) Light is on then it's a good thing indicating that the Antilock brakes sensor is in good condition. I too have a 2002 mountaineer
the bulb has fused or broken or the electronics need fixing
check the brake light switch
On a 2002 Mercury Mountaineer : ( DOT 3 brake fluid meeting Ford specification ESA-M6C25-A ) According to the Owner Guide
According to the 2003 Mercury Mountaineer Owner Guide : ( DOT 3 brake fluid , meeting Ford specification ESA-M6C25-A )
DOT 3 brake fluid
DOT ( 3 ) brake fluid , if that is what you are asking ?
It should have only a fuse and a switch, no relay. Check ALL of the fuses with a test light.
check brake fluid level.
In CT USA, a garage would charge 1 hour labor to replace the rear brake pads on that Mountaineer.