There's two factors involved in the wiper rest position.

The position of the wiper arm on the shaft and the operation of the switch
that detects "rest" position and interrupts the motor drive.

Those wiper arms are designed to shear the soft metal insert to save the
motor spline.  So be careful in tightening them.  The splines are designed
to "deform" or "bite into" the insert.  and the insert is tapered and of the
proper shape to allow this.

When you tighten it down too hard, the insert deforms to it's limit and if
jammed will stall and burn out the wiper motor.  (Ice on the wipers can help
with this).

If the arm is loose on the shaft, you should be able to tell by gently
moving it.  Remove the arm.  Clean out the splines with a stiff brush or a
small sharp implement (I use a small steel brush)  until no residual metal
from the arm remains.  Then closely examine the arm by fitting it by hand
over the spline.  It should not "bottom out"  If it does, adding addtional
tightening torque to the installation will not solve the problem.

Now with the arm off, mark the spline and test the rest switch several times
to ensure it always returns to the same position by turnining on and off the
wipers and examining the marked spline's angle.  If they aren't the same
each time the rest switch has failed and the problem is not with the arm by
itself.

If they are the same, remove the blade holder from the arm.  Reinstall the
arm, taking care not to use too much force on the fastener.  Now check the
operation again.  This configuration has the minimum amount of drag (none)
for ensuring the proper return to rest of the arm.  If it seems to return to
the same position each time, re-install the wiper blade and retest.  This
configuration has resistance to motion by the dry sliding friction and if
the insert is too worn to allow the splines to properly deform the metal
then the wipers will seem to stop in non-rest positions although we know the
rest switch is working properly.  Time for a new arm or use a soft metal
shim in the shape of the inside of the insert as a temporary fix. Remember,
the arm is designed to fail to protect the motor and spline.

pensioner