Looks like a packet-radio interface that would have been connected to a serial display or terminal mounted in the cab. The two RJ11 connectors on the left going to a keyboard, display and possibly printer. This would provide the driver with simple text based communication to a central control room. Used by dispatch and delivery guys, engineers etc., whilst on the road.

Hardly covert as it would have required an externally mounted antenna about 6-12" long!

My guess - guy's vehicle is an old company van or truck, he's bought it second-hand and it's been left there by the by the previous owners.