The surface on tempered glass is harder than the core and is in a state of tension. Like a balloon if you cut a hole in it, it will "pop". Even if you successfully grind a hole through tempered glass it wont last long before it pops. You cant even trim tempered glass for this reason, the tempered surface is continuous on all edges and surfaces. Custom glass is the solution unless you can move the wiper into a nearby body panel instead.