You don't really, you would replace the entire linkage. Best place to get it from and be correct the first time would be your dealer.
Its a dealership part
Check eBay for the plastic linkage bushings. They are not available from the Ford dealers.
The linkage bushings are worn/missing/broken or the wiper arms are not timed properly to the wiper transmissions.
Rockauto.com has them and so does O'Reilly's auto parts A Chrysler dealer should be able to get them.
Remove the old wiper and replace with a new one
Below
The 1999 Nissan windshield wiper linkage is held in place by three retaining nuts. Remove the retaining nuts and the linkage will come free. Replace the linkage with new linkage.
The wiper linkage is bad. Or the wiper arm stud bushings (where the arms attach to the cowl) are rotted out.
If one wants to replace a wiper linkage there are a few steps involved. Pull the wiper arm away from the windshield until the arm locks in vertical position. Unbolt the nut that holds the wiper arm to the linkage and pull the wiper arm off the linkage. Open the hood and unbolt the wiper motor and slide the motor off of the linkage. One then will remove the wiper linkage and install a new one.
I have the same problem and the engineers at ford expect you to replace the entire wiper assembly if the cap over the linkage ball breaks... every other maker has this as a replaceable part.. whats up??? Pick up replacement cup-and-ball linkage bushings from local O'Rielly Auto Parts or online for less than $10. Made by Dorman part number 49451, brand number YL3Z 17508-AB. Replacement instructions described for 2000 Expedition which uses same part here: http://voices.yahoo.com/replacing-wiper-arm-bushings-2000-ford-5389617.html
Linkage bushings on wiper arms coming off back side of wiper motor under cowling
LMAO, you have one too? Are you certain the motor is bad? The linkage can come apart because of perished bushings. If the bushing fails, the motor will turn, but the wipers won't move. Check this before you buy a new motor that you can't return.To remove the motor, disconnect the power lead. Remove the three wiper motor mounting bolts. A gearwrench is the best tool to reach this tight spot. You may be able to pull the motor back enough to unfasten the crank. The crank is held by one nut. If you can't pull the motor back far enough and reach this nut, you will have to remove the upper cowl to reach the nut. To remove the cowl, remove the wiper arms. There's a retainer catch on the back side of the wiper arm that needs to be released as you pull up. You may need to use some PB blaster to free corrosion to get these loose. With the wiper arms off, remove the eight or so sheet metal screws. With the cowl removed you can reach the nut holding the crank. This is a good time to replace all of the wiper bushings, Doorman p/n 49440. These are plastic and on a 1982 they will be dry-rotted. These bushings will fail when it's raining and you're a long way from home, so replace them while you're replacing the motor. On mine, all four crumbled and fell apart.To remove them pry the joint apart. Remove the wiper linkage assembly and pivot posts as a unit. Make notes which side of each linkage arm receives the bushing. Each wiper pivot post has three mounting bolts. With the linkage out on the work bench, install bushings into the linkage arms using a socket for backup. Hammer them in until they latch into place. Once the bushings are installed in their holes, hammer the pivots into the bushing. These will also latch. Use a little bit of grease to ease the assembly. With all new bushings installed in the wiper linkage assembly, install the linkage assembly (all three links and the two posts) through the access holes starting on the passenger side. Slide it through into position and replace the three bolts at each wiper post. Fasten the linkage to the crank arm. Replace the cowl and install the wiper arms.