The wire snakes down the gear shifter under the car onto the transmission and finally to the solenoid on the driver's side of the transmission. There is a plastic electrical connector thingy too...that might be the problem also. Bottom line, if the wire is grounding something metal, then it may be affecting the solenoid. This solenoid is normally ENERGIZED. If the solenoid is de-energized, then the arrow on the dash will be lit and the car will downshift (stay) to third gear. In order to have a 4th gear, the solenoid must be energized.

On the other hand, if you want to permanently disable the OD solenoid feature, there's a quick fix for these automatic transmissions for that. It involves pulling the solenoid off and carving out a small channel for the transmission oil to pass thru. Then the solenoid can be re-installed, remain de-energized, and you'll still have 4th gear. However, this permanent fix will not allow you use the little button on the shifter anymore.

If you're sure it's not the OD solenoid and you're sure the wire is intact, then it's prolly the solenoid itself.
Cabbie2169