I notice from the '99 power distribution diagrams that all of those faulty lamps are fed with 12 volts via 'Switched System Power Relay 2', so a faulty relay could cause the problem - a quick and easy check would be to swap relays SSP1 and SSP2, assuming them to be identical.
According to the owner's manual they're in the engine compartment fuse box, with Relay #301 being SSP2, and relay #302 being SSP1. The layout should look like this:
'99 Engine compartment fuse box layout.
Fingers crossed that it's just a faulty relay, because if it isn't then it will mean delving into the wiring, which I'm sure you'd rather avoid. These are the power distribution diagrams by the way, and you can see what I mean on diagram #4:
'99 Windstar power distribution diagrams:
1 -
2 -
3 -
4