Under the cowl in front of the windsheild on the right side.
My '85 Grad had intermitantly slow, brutally slow, and plain slow til I opened things up and tinkered the linkage a bit.
At one point I marked the position of the adjustable part of the left end and uncoupled it then turned on the motor (you need to hold the linkage up or it'll beat viciously against about everything in there **WATCH YER FINNERS!!**) and was really surprised at how quickly the drive arm could move on it's own.
The most tedious part is taking the cowl off then removing the duct then putting it back together with the duct removed so you can adjust as it has to be on the car and operational to tweak at it properly.
I has to spend about an hour making minute adjustments to the left end when putting it back together, but the net result was moderately faster movement overall when it was done. (we're not talking stupid fast here, but the slow speed is on par with any car out there slow speed and the fast is just a bit slower than what one would call 'normal fast' and the volt meter I have installed shows a slightly lesser drop in V when the intermitants kick indicating that it's taking less initial draw volts due to less mechanical resistance)
My best guess is that over time the linkage bends ever so slightly through general air resistance as the wipers are moving at highway speeds, (put you hand out the side when movign at speed, then imagine a wiper and arm doing this for a couple decades), somone just tugging at them when washing the car, (hey, that's where all the bugs die, right
), or even 'pedestrians' who can't resist touching things as they walk by in combination with with wear on the plastic pivot bushes over time causes things to get out of whack.
Another thing that helped ever so slightly was switching over to silicon wiper blades.
Again, none of this made them brutally fast, but the intermittant and slow pace of the wipers is definitely improved and almost match a modern car while the fast is noticably a bit quicker but slower than what one would expect on a modern car.