IF the motor is supposed to stop when it reaches the top, then something has to tell it to stop.
The motor would not do it by its self.
On that old a machine, I doubt it has anything to stop it, but I could be wrong.
I have been unable to find a schematic on that system.
On some of the new auto down windows, they either have a limit switch, or a relay in series with the motor winding.
When the motor is winding the window down, it is not drawing enough current to cause the relay to pull in.
When it stalls out, it does and the relay trips off the automatic down part.
Having said that, if you does have a limit switch, then that might be a source of the problem.
Since you have eliminated the dash switch, its almost got to be in the tailgate.
There has to be a hot wire going back there for the outside switch to work.
Odds are its going to be in that switch.