Janie,
When you remove the face plate on the left end of a 66 you expose everything that moves as far as the needle bar and presser foot. It will look like this, more or less. (This is a 99, but they are almost identical.)
The main shaft is oil-able via oil holes all across the top.
You can access the connecting rods through the big inspection hole under the big round plate on the back of the pillar.
That is about all you can open up on a 66.
Flood the parts in the left end with oil and or Liquid Wrench Penetrating Oil, let it soak and wiggle it every so often.
Also flood the bobbin parts too. If they are jammed up it will mess up everything else.
Joe