I encountered this VERY problem with my 03 WRX. The bushing on the left-side ball link was worn out, meanwhile the right-side link was in perfect condition no slack/slop whatsoever.
I took the free/cheap approach to fix this problem as well, as I did not want to fork out any cash for the whole assembly just to fix a bushing problem.
I ended up using a brass washer and two stainless steel shims to fill the gap ( I also applied grease for lubrication). I then re-installed the plastic cap on the ball joint and there is no play whatsoever. I just did this over the weekend, but I don't have any doubts about longevity. I feel that the brass washer and SS Shims will hold up much better than the cheap plastic bit.
My question to everyone that has dealt with this is: Was it the left or right ball joint plastic washer that had the problem? I'd like to see if its a common problem with the same side.
I also feel like Subaru sold-out on the wiper mechanism by using these cheap plastic parts. Perhaps using a bronze spacer and/or a sealed bearing would have worked better. From my research this is a VERY common problem among the Impreza groups, but I found the washer solution here first and that's why I became a member of this forum to give my gratitude and support. Comments and questions are welcomed.
Today, I took apart the wipers on our 98S in search of the excess play in them. I could easily move the wipers up and down 3" when parked. They worked OK, but this play didn’t seem right.
I discovered the wiper motor arm and wiper link joint had a lot of play in it. The wiper link arm had slid off the white plastic bushing and was free floating off the bushing. I think that the bottom black rubber bushing has weakened with age and no longer holds it in place. Or possibly the bushing has just plain worn out. This week I will try and order new parts to see how they are any different and update this post.
My solution for now is to add a washer/spacer . By adding this the wiper link arm now stays on the bushing as it is supposed to do. The wipers now have no play in them just like new. There is nothing hard in this procedure, definitely a DIY item.
Wiper Assembly PDF