Ford Motor Co. has extended brake rotor and lining warranties for 1989-90 Ford Thunderbird and Mercury Cougar cars to 24 months, 24,000 miles or until May 31, 1991, whichever comes last.

The company said uneven rotor wear could cause shaking in the steering wheel and vehicle vibration. About 407,000 cars are covered by the extension.Owners of the cars are being notified by mail.