386 Peterbilt wiper linkage

I don't know if the 386 is the same as a 379, on a 379 you take the glove box out and can get to the right hand wiper linkage.

 

I put the Bosch Ion blades on my 389. Not cheap but work very well.

 

I would pull the wiper arm off the spline. Turn on the wipers and see if the spline is moving. The arm may not come off very easy. Spray with penetrating oil and tap the connection. When you put it back together put some never seize on the spline and threads. Future reference do not use your wipers to clear snow and ice off your windshield when you start your day. Flip the hood, climb up there with a snow brush and clean everything off including the wipers. That stuff is a pain to work on so the plan should be to baby it so you do not have your current problem. If the spline is moving with the arm off, I would take a pick and clean both splines on the cab and on the arm. Put it back together and see how it works. If it does not want to work I would try replacing the wiper arm before I tore the spline linkage apart.