If it's the same as mine, it's a led light and you have 2 replace the power converter, there's no bulb. $250. I rigged mine up 2 use a 12 volt lamp.
Go to dealer...and they will change them to early.
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 ?
check brake fluid level.
To change the height brake light on a 1997 Mercury Mountaineer, first, locate the brake light assembly at the rear of the vehicle. Remove the screws or clips securing the assembly and carefully pull it out. Disconnect the wiring harness, then replace the old bulb with a new one, ensuring it's properly seated. Reconnect the wiring, reassemble the brake light housing, and test to confirm it’s functioning correctly.
In CT USA, a garage would charge 1 hour labor to replace the rear brake pads on that Mountaineer.
According to the 2000 Mercury Mountaineer Owner Guide : ( Yes , DOT 3 brake fluid , meeting Ford specification ESA-M6C25-A )
Should be common to the brake light circuit
Disc brakes ( with small brake shoes in the back of the rotors for your parking / emergency brake )
bad switch or possibly stuck switch