WHat i did was to ask the beggars or the countesses maid and take a secret passage in the castle basement. The switch is in a barrel against the far wall. Just walk through the passage and you will be in the private quarters. Just be careful because sometimes the countess will randomley come through there and attak you. At 10-11 o'clock the countess will be asleep but not for long. The guard should also be gone if not wait or if you have an invisbillity spell use that and go into the jewlery box. quickly run out the same way and you should be done.
Murphy's Law: It is impossible to make anything foolproof because fools are so ingenious.