Located on the underside (usually) of the left coolant pipe, near the back. OK, you know that. Mine fell out, you may have read that. It fell out because it was the wrong one plus it didn't have the clip, that wouldn't have fit it anyway, and it was held on with silicone seal which ultimately let go. Here's the point, the stock clip is really pathetic, it has to rust out in time. The real talents in our midst all agree that the best way to keep it in place is with a nylon tie. I'm suggesting you put a tie around yours, even with the clip, you do it from the top, in the engine compartment, don't even have to crawl under. If it falls out, much coolant immediately leaves your cooling system and the otterstat doesn't know that your system is heating up because it's hanging by it's wires below the engine where it's cool. Your fans don't come on because the otterstat didn't tell them to. You can figure the rest of what might happen. You could have saved it with a 25 cent tie. Cheap insurance?