Replace the speedometer cable. Better yet, replace the car. I had this problem in my 94 acclaim. Somehow it had gotten severed and no new part was needed. Take it to a mechanic you trust and all it will cost you is labor. No need to replace the car.
The timing belt on a 1991 Plymouth Acclaim is replaced by removing the engine accessories, removing the timing cover, and taking pressure off the idler pulley. The belt can then be slipped off the sprockets, a new belt put in place, and the tension restored.
In my manual it will tell you (1993 Plymouth acclaim)crasp firmly both sides of center brake light housing and pull straight out,turn the bulb,pull it out,replace bulb(check bulb number)had to give it a pretty good tug,but that's it.