Speedometer not working?

The speedometer and odometer on my husband's 1997 Nissan pickup was sticking a few months ago, but if you tapped on the plastic in the cab, it would start to work. Now neither work at all. We want to sell the vehicle, which only has about 123,000 miles on it, but are concerned that prospective buyers may think it has much more mileage on it due to its age. On the other hand, we don't want an expensive repair. What causes the problem? Can we repair it ourselves (new cable or rehook up the original)? If not, how expensive a repair should it be? Thank you.