Q: No brake lights 1990 Mercury Grand Marquis

Hi there - check the fuses for the brake lights, the brake light bulbs, and the ground for the brake lights (should be OK since other lights work, but....). If these check out correctly, check for power to the brake light switch -ignition in the "Run" position - then check for 12v on the "outbound" side of the switch with the brake pedal pressed. If there is 12 volts leaving the brake light switch, there is a wiring problem going to the brake lights themselves. If you need assistance tracking this problem down, I would suggest a wiring inspection by a mobile, professional mechanic, such as one from YourMechanic, who will come to your location, diagnose this problem, give you an accurate assessment of damage and cost estimate for repairs.