If the bearings and shaft are galled, there is not any lubrication on
the market that will cure this problem.
Removing the armature, and bearings and burnishing them to a mirror
finish might solve the problem.
But the time and energy that this would take to accomplish would be
ridiculous.
The reason the motor will run off a direct 12 volts is that it's drawing
so many amps it won't run with all the wire and switches proceeding it.
The wiring is eating up the required amperage prior to the motor.
Or the armature is bad.
Buy a new motor.
The rear heater is easy.
Try and replace the front one.
Then report how bad the back one is to replace.

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For those of you who've been keeping old blower's alive: I remember
plenty of posts about lubricating blower motors, but don't recall
anything about what products you used and where in the motor you applied
it.
In my case, it's a rear blower that only runs on high speed after
reinstallation. Feeding voltage directly from the battery to each of the
terminals it does ok, so I'm probably looking at an electrical loss
somewhere, but digging the thing out is enough of a pain that I wouldn't
want to do it again any time soon.
Thanks,
Matt Sutton
88 GL