Where is the thermostat located on a 1998 Chevy Malibu 3.1L?

The thermostat on a 1998 Chevy Malibu 3.1L is located on the engine, at the lower intake manifold. It's not easily accessible; you'll need to remove some components to get to it. Specifically, you'll likely need to remove some hoses and possibly the lower intake manifold itself or at least part of it depending on the exact access. This is a job that generally requires some mechanical aptitude and tools.