Chances are you may have a wheel speed sensor that has failed on your 1999 Mercury Grand Marquis. When the warning lights are on fault code(s) should be stored in the control module, the anit-lock brake and traction control systems will also be disabled. When these systems are disabled you will not notice any difference in performance or braking during normal driving conditions. These system only operated under extreme adverse driving conditions when wheel slip on braking or acceleration is present. Having the stored fault codes in the ABS control read and diagnosed should lead to the cause of your warning lights being on. It most cases a scan to is necessary to access the stored fault codes.
A collapsed brake hose can cause that. It will allow the fluid to go to the caliper but not return properly. A dragging front brake will activate the traction control and set off an ABS service warning. If you remove a brake hose and can't blow through it, it's junk. In the old days a collapsed brake hose would just eat up the wheel's brake pads. It's an $18.00 part.