I have the shop manual and the body by fisher manual but neither of these call out part numbers.
My 2 speed wiper switch died and I’m having issues tracking one down. Any advice on how to find part numbers and/or compatible replacements?
I would look first at all the Buicks to see if others have your part. You would need to find vehicles that share your parts. Then, it is a GM vehicle. Also, not always, the size of the vehicle or chassis, because GM typically would put the same part in multiple vehicles in order for lowering cost in production. Third, many of the older parts are repairable because they have metal. Even the plastic stuff, can be repaired now with all the methods available. Last, I've found some of my parts at vintage car swap meets for close to nothing in price. Many of the sellers don't even know what the part goes on and just put them in boxes mixed altogether.
You need the Buick Master Chassis Parts and Buick Master Body Parts books to find these numbers. Those are where Bob got the 1993344 number he posted for you - from the Buick Master Body Parts book.
These are almost always available on eBay, either originals or sometimes reprints.
The part number for the switch came from the Body parts book. The Group number for that switch is 10.xxx (10.163 to be exact), since the Chassis book covers Groups 0.xxx through 9.xxx, and Body book covers Groups 10.xxx through 15.xxx Each type of part has its' own 3-digit suffix to the main Group number - 10.159 is wiper motors, 10.163 is the switch, 10.164 is the knob for the switch, etc, etc.
For a 1966 car, if you want to buy these, I would maybe choose either 1967 or 1968 print dates for these books. That way you can see if a part you are looking for fits a car a year or two later than yours. A lot of mechanical parts (and some body parts) for a 1966 B-body car fit through 1970. Even 1970 versions would be good, but they are somewhat scarcer (and more expensive).