It's usually found either on the transmission itself or on the steering column, near the ignition switch. However, some trucks might have it integrated into the transmission's control module.
To find it, you'll need to consult your truck's repair manual or search online for a schematic specifically for your year, make, model, and transmission type (e.g., "2004 Ford F-150 4R70W neutral safety switch location"). Using the wrong information could lead to serious problems.
Caution: Working on the neutral safety switch often involves working with the electrical system and potentially the transmission. If you're not comfortable with this type of repair, it's best to take your truck to a qualified mechanic.
