Bongo Fury

I have found a little black/grey box behind the dashboard instrument cluster (trying to sort out old switches). Three out of 5 of the wires are connected (the red +12v, the blue mpg signal output and green earth). The yellow KMG signal input has been cut and connected to a blue and white wire in the loom, and the grey ECU output doesn’t seem to be connected to anything.

Is this a converter for the speedo and should the grey wire be connected - if so, to where?

The colour coding will be specific to individual makes, most likely Red will be +12v from the Ignition feed, Green most likely to be Earth/0v connection the other pair of wires will be the Input and the Output normally its a case of the original input wire from the sensor is intercepted either in the loom or on the PCB and the module Input and Output placed connected to the intercepted connection. You might be able to get some information from any label on the module by googling for "speedo chip".