On 99s, the nuts holding the wiper arms to their respective posts are 15mm. Sometimes they can loosen. If they still turn with the arms removed, you can try to resnug, but i usually slip a thin lock washer under the nut before tightening. Others have used blue loc-tite with some success. If the stud posts dont move with the arms removed and wipers activated, time to look at the linkage and motor under the cowling as stated above. I just finished earlier today replacing a broken white linkage bushing on an 04 pontiac grand am. If this turns out to be your issue, arm linkage bushings are available in assortment packs in the Help! section of most parts stores.