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.
To replace the wiper linkage bearing on a 1985 Chrysler 5th Avenue, first, remove the wiper arms by loosening the nuts and lifting them off the shafts. Next, detach the wiper motor and linkage assembly from the vehicle by unscrewing the mounting bolts. Carefully remove the old bearing from the linkage, then insert the new bearing, ensuring it's properly seated. Reassemble the linkage and wiper motor, and reattach the wiper arms to complete the repair.
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