Where's the coolant temp sending unit in E350 ClubWagon

I’m trying to figure out why the temperature gauge doesn’t work, but I haven’t been able to locate the coolant temperature sending unit on my 1991 351 Ford E350 Club Wagon. Where is it?

The sending unit is located on the left side of the intake manifold, near the distributor. It has a single wire plugged in.

Tester