overdrive switch problems

I have a 1999 Ford Expedition w/5.4. Does anyone have a solution for the overdrive switch that continuously blows the fuse for this circuit? I had a transmission shop replace the actual switch on the shift selector. The only answer I get from the "pros" is that they will have to track down the whole wiring system from the gear selector through the steering column and down to the transmission.
The vehicle runs fine and shifts into overdrive however without the switch/button working I can not tow my boat due to its weight. Is there a simple fix out there? Thanks.