Fords are quite known for this problem, i have a 1998 windstar and my ABS and brake lights are on 24/7 even though i just had new brakes and such put on they still are stuck on there. for some reason ford has issues with their sensors. If you have had your brakes done recently then i wouldnt worry about it but if you still have original brakes on, i would get them checked out just in case. some places check for free