This is something I never figured out in King's Quest 3, To Heir is Human- the wizard Mananan had a safe in his study that I assumed held a magic wand, but I could never unlock it to find out for sure- how did one do it (if it was even possible)?
It's been a long time since I played this, so I had to turn to GameFAQs on this.
According to that FAQ, you open the safe with the "brass key", which is in Mananan's room. The FAQ differs from my memory a bit though, as it says:
LOOK ON TOP OF CLOSET to find a small brass key. OPEN CLOSET and LOOK BEHIND CLOTHES to discover an ancient map.
I mean, it's been 15 years since I've played, so the FAQ is probably correct, but I thought I remembered it being the map on the top of the closest and the key that was behind the clothes. Either way, check both. It'll be in one of those two spots.