Where is the horn on a 1996 e350 van?

The horn on a 1996 Mercedes-Benz E350 van (assuming you mean the Sprinter-based van, as there wasn't a car-based E-Class van in that year) is typically located behind the front grille. You'll likely need to remove some parts of the grille or bumper to access it. It's not easily visible or accessible without some disassembly.