change the blower motor the bearings have gone bad in it
Look behind the glove box. under dash.
maybe your blower motor relay is stuck or burned together ???
A bad heater blower motor resistor would effect one or more blower speeds other than high.
The Mercury Cougar blower motor is located on the firewall in the engine compartment. The blower motor can be seen on the passenger side of the firewall.
Under dash at RH side, mounted with 3 bolts. R&I about 3 minutes.
location of relay blower motor 1997 Mercury mystique
If your blower is working on some speeds but not all, more than likely the problem is a defective Blower Motor Resistor Pack. The switch rarely goes bad. Just a thought.
No repairing those blower motors, replace it with a new one.
Without knowing what model year the questioner is referring to...Budget somewhere around $35-60 for a blower motor - likely to be closer to the lower endSee "Related Questions" below for more
front of the motor
It could be a thermostat that is stuck open or maybe a bigger problem...like the blend door is either jammed or broken as well.
According to the 1999 Mercury Sable Owner Guide : ( 5W-30 )