You have to figure out why it is not working -- you have the car. It is either the spedo in the cluster or the spedometer cable. Your '86 should also have a "black box" that counts miles to signal when the O2 sensor needs replacement. The cable "passes through the box and there is an electrical connection on the box. The box is along the firewall in the engine compartment half hidden by the brake servo and master cylinder assembly.
I usually delete the black box (I have had problems with them failing.) and replace the split (to the black box and from the black box) speedometer cable with one cable for the whole length. It is a standard replacement part -- probably less than $20 for a new one. If the box is nowhere to be found perhaps someone alredy deleted it. The cable goes from the transmission to the back of the cluster. Although it is a tight fit, you can usually reach up under the dash to undo the screw on connector at the cluster. You might have to remove a part or two of the lower dash trim to assist with the reach and visability up under the dash . The transmission end of the cable inserts into the transmission, has a gasket to prevent transmission fluid leakage and is secured to the transmission with a with a small bolt or screw.
If a new cable doesn't fix the problem -- replace the cluster or the speedometer part of the cluster. FR