Do you have access to a multimeter?
You could check the circuit for a short to ground but you would need the fuse, bulbs, and buzzer out and the dash lights and switch disconnected. I know it's a faf but you are starting from a good point. If there is no short measuring from the circuit (feed to sidelight bulb would be easiest) to ground reconnect the light switch.
Recheck for a short with the switch in the sidelight position still with the fuse out. If you now read a short circuit to ground yes its probably your switch.
If however you still have no short to ground keep reconnecting the parts one by one, rechecking for shorts at each stage. Sooner or later you will connect the part causing the problem and Bobs your uncle.
If you dont have a meter you can follow the same procedure using a fuse and someone spotting it to tell you when it blows.
If you do have a short with everything out of circuit then it gets a bit trickier to find but at least you know where to start looking.
John