The oil pressure sending unit on a 1999 Lincoln Town Car is located on the
engine block, near the oil filter. It's not always easy to see without removing some components. You'll likely need to consult a repair manual with diagrams specific to your engine (either a 4.6L or a 4.6L DOHC depending on your trim level) to pinpoint the exact location.
Because it's near the oil filter, getting to it will probably involve dealing with some hoses and possibly the filter itself. Always remember to disconnect the battery negative terminal before working on any electrical components near the engine.
