Where is the vacuum modulator on a 85 buick regal?

The vacuum modulator on a 1985 Buick Regal is located on the transmission itself. It's not easily accessible without some disassembly. You'll find it on the driver's side of the transmission, typically near where the transmission lines connect. It's a small, cylindrical device with vacuum lines connected to it.

However, precise location can vary slightly depending on the specific transmission type (TH200-4R is common but not guaranteed). A repair manual specific to your Regal's transmission is the best resource for pinpointing its exact location. A diagram in the manual will be far more helpful than a general description.