Where is starter solenoid on 1991 buick regal 3.8 engine?

The starter solenoid on a 1991 Buick Regal with a 3.8L engine is typically mounted on the starter motor itself. The starter motor is located on the engine block, near the transmission bellhousing, on the passenger side (right side as you're sitting in the car).

You won't see a separate, standalone solenoid like on some older cars. It's integrated into the starter. To find it, you'll need to locate the starter motor and then you'll see the solenoid as part of its housing. It will usually have a large cable going to the battery and a smaller cable going to the ignition system.