Screw in fan and run continuously for at least 2 weeks maybe longer depending on temp in barn or storage area and outside temps.  Not so much for drying but to get it completely cooled down. 

Run periodically afterwards and check temp deep in wagon.

Can make a simple probe 2 pieces of copper pipe one slides inside the other 1/2" and 3/4"

Drill several rows of holes in larger pipe and hammer end shut to make a flat blade. Insert 1/2" pipe in larger one which blocks the holes. Push this into the grain. As you pull out the inner pipe it opens the holes where wheat can run in.

You can pull from different levels in the wagon to test grain temperature.