Just bought my first Subaru yesterday and it's a 1989 GL. Unfortunately, all 4 Windows seem to be weak and barely roll up and down. What do you recommend I do to attack this issue? Thanks!

One site that I found recommended spraying silicone spray into the tracks. A bit messy, I did and it offered a little relief but not like new.

It could be old dried out grease in the mechs, or high resistance in wireing / connector/s.  Could be both.

 

If it's the grease, clean out the old, put in new.

Check all the connectors for signs of heat./ looseness.

and the power/ground splice below the carpet in the passenger footwell if cleaning the hardened 30 year old grease out of the tracks in the door does nothing for window speed. 

 

spray on white lithium is fine. i used ceramic brake parts lubricant called ceramlub because i got a tub for free at a trade show and i like its consistency. its just window tracks, so don't overthink it, just make sure its lubed.