Installing warning lights may disable brake shift interlock
Fleets that manage Ford police, emergency or municipal vehicles (Crown Victoria, Exepedition, etc.) may have had their vehicles modified to flash the brake lamps whenever the police lights or other aftermarket lights are turned on, which may disable the brake shift interlock.
The brake shift interlock feature prevents the vehicle from being shifted from Park unless the brake pedal is depressed. Disabling of the brake shift interlock may enable the operator to inadvertently apply the accelerator instead of the brake pedal and simultaneously shift from Park to a drive gear.
Shop operations that manage police vehicles (in addition to municipal or corporate vehicles) need to be aware that any vehicle that has been modified with a connection to the brake circuit, or that electrically interfaces with the brake lamps, those connections should be disconnected immediately, and the emergency lighting system should be modified in a manner that does not interfere with normal service operation or the brake shift interlock. In addition, if the high-mount stop lamp flashes when the police lights are on, the modification does not conform with the Federal Motor Vehicle Safety Standards (FMVSS 108) which requires that the high-mount stoplamp only illuminates when the brakes are applied.
Installation of warning lights should only be performed with a completely separate electrical system, without connection to any existing vehicle wiring. Connection of aftermarket electrical equipment into the brake lamp circuit or any other circuit which is connected to the powertrain control module, anti-lock brake computer, air bag system or any other vehicle system will cause a vehicle malfunction.
According to Ford, this caution applies to all Ford, Lincoln and Mercury vehicles.