Hi, Just so you know going in, these are not easy to do and are not for the mechanically challenged. So, be sure you are up to the task. I have no idea as to your ability, so thought I should mention that just in case. The information on location and how to replace the thermostat is copied below for you from the Pontiac service manual. Thanks, Bob
Caution: Unless directed otherwise, the ignition and start switch must be in the OFF or LOCK position, and all electrical loads must be OFF before servicing any electrical component. Disconnect the negative battery cable to prevent an electrical spark should a tool or equipment come in contact with an exposed electrical terminal. Failure to follow these precautions may result in personal injury and/or damage to the vehicle or its components.
Notice: When adding coolant, it is important that you use GM Goodwrench DEX-COOLTM coolant. If coolant other than DEX-COOLTM is added to the system, the engine coolant will require change sooner: at 50 000 km (30,000 miles) or 24 months.
Notice: Use the correct fastener in the correct location. Replacement fasteners must be the correct part number for that application. Fasteners requiring replacement or fasteners requiring the use of thread locking compound or sealant are identified in the service procedure. Do not use paints, lubricants, or corrosion inhibitors on fasteners or fastener joint surfaces unless specified. These coatings affect fastener torque and joint clamping force and may damage the fastener. Use the correct tightening sequence and specifications when installing fasteners in order to avoid damage to parts and systems.
Tighten Tighten the bolt to 26 N·m (19 lb ft).
Tighten Tighten the bolt to 14 N·m (124 lb in).
Tighten Tighten the bolt to 16 N·m (12 lb ft).