2002 Mercury Sable

I had the same issue but it was intermittent. The dealership (luckily it was under warranty) spent a few hundred on replacing what you've already done trying to fix it. You could hear a clicking noise under the dash near the fusebox when the button was pushed. Finally they listened to me and installed a new relay, the problem has never returned since. The part is expensive so a junkyard car ford or otherwise may be the way to go. I would also check the fuses if you already havent done this.