One problem related to back up alarm has been reported for the 1999 Ford Windstar. The most recently reported issues are listed below.
The reverse sensing system at the present time is inoperative. It has been back to Ford 5 or 6 times for repair. It seems to fail after a heavy rain. I was involved in an accident 1 time when I was backing out of a parking space and backed into a vehicle that was passing behind me which I didn't see. There has been several other times when I relied on the sensor to warn me if someone was walking behind the van especially someone that is short, I'm unable to see and depended on the reverse sensor to warn me. Each time I went back to the dealer until the warranty ran out to have it repaired. The repair dept told me that it is poorly designed. As soon as the wires to the system got wet it would fail. They changed the harness and it would work until it got wet.