I replaced my 66 wiper motor with a rebuilt 69 motor. It really sped things up.  I added a intermittent switch from a 79 F100 and that is one of the best mods I ever made.

Do you have a 3g alternator?  The extra voltage helps speed things up.

I have the factory two speed wiper motor and installed a variable intermittent system from a late 70`s pickup several years ago. It worked great going home from Ft Walton beach when the hurricane came thru but it was beginning to slow down then. I didn’t install the G3 alternator till after we got home. Now it is real slow.

Bullet Bob wrote:

Sounds like a voltage issue. As Dan has suggested, check voltage with the motor disconnected, then check with the motor connected and operating.  I suspect crappy/dirty/corroded connection (s) somewhere.  Maybe at the firewall where cabin power comes in.  You can run a hot jumper directly from the battery to the motor and see it works better, then you'll know if it's related to a poor connection.

Ditto BB's response.  You may want to try one of the products offered from Deoxit, I've it on many of the OEM connections on my 65 with success. I have "Deoxit, #5L-25CA (5% Solution)".  (That cost a lot less when I purchased many years ago)
https://caig.com/deoxit-d-series/
 

MS - "kind of pricy" is right. 

BobE wrote:

Bullet Bob wrote:

Sounds like a voltage issue. As Dan has suggested, check voltage with the motor disconnected, then check with the motor connected and operating.  I suspect crappy/dirty/corroded connection (s) somewhere.  Maybe at the firewall where cabin power comes in.  You can run a hot jumper directly from the battery to the motor and see it works better, then you'll know if it's related to a poor connection.

Ditto BB's response.  You may want to try one of the products offered from Deoxit, I've it on many of the OEM connections on my 65 with success. I have "Deoxit, #5L-25CA (5% Solution)".  (That cost a lot less when I purchased many years ago)
https://caig.com/deoxit-d-series/
 

I don't think measuring the voltage with the motor disconnected will tell you anything. Even if there is a bad connection prior to the motor, with the motor unplugged it will show full voltage. This could make you falsely think there is not a problem.  Voltage reading near the motor with the motor connected and running should be near full voltage. I like Bob's idea of a jumper from battery to the wiper motor. If it works full speed then you probably have a bad connection in your wiring. If it still runs slow then either a bad motor as Rufus said or bad ground.