It's the box about 1" by 1" by 2" deep on the engine side of the wirewall ahead of the driver. It's usually a silver box with red top and a clear plastic lid over a blade type fuse. FastLane has a picture of it.

It can also be a faulty connection at the flywheel rpm sensor or a faulty flywheel rpm sensor.

Sixto
95 S420
87 300SDL