I've performed this procedure on my 98 Outback as well as on my 03 Impreza. They're almost identical.
To start, remove the plastic cover on the wiper arm where it comes out of the glass....(pop it off with a screwdriver).The small nut under it is a 10mm. Chances are it's a bit rusty so you might want to give it a shot of liquid wrench or something.
Remove the round plastic cover and then remove the large nut.
Now, from the inside, remove the interior panel to reveal the motor.
Unplug the yellowish wire connector then remove the 3 bolts,(10mm). The motor should fall out.
Remove the small screws as shown in pict 01. Remove the back carefully as to not let the big plastic gear fall out. Take note of the position of all the parts.
Before removing the shaft, it'll probably be rusty like the one in the picture...soak it with PB blaster and go for a beer.
I used a clamp to remove the shaft in mine...as in pic # 02.
Once the shaft is out, clean it up real well with sandpaper...The pics show mine before and after.
Clean the hole where the shaft goes with a piece of rolled up sandpaper then with a Q-tip.
Grease the shaft and the hole with Brake Caliper grease and re-assemble. Make sure everything is lined up like in the picture.
If it's assembled wrong, the wiper will rotate in the wrong direction.
Put the unit back in and try it before putting the panel back on, just to make sure it works.