What is the red light that comes on every now and then warning center just above passenger sliding door your 1991 Ford Aerostar?

That red light above the passenger sliding door in your 1991 Ford Aerostar is almost certainly the sliding door ajar warning light. It illuminates when the passenger side sliding door isn't fully closed and latched. Sometimes a slight misalignment or a problem with the latch mechanism can cause it to illuminate even when the door seems closed.