Unplug the thin coax cable from the Trimble GSP receiver unit, and connect it to the small coax socket on the back of the MkIV nav unit.

Also, you should totally unplug the Trimble GPS receiver as it is no longer required - the GPS receiver is now built in to the nav unit.

If I were you, I'd remove the Trimble GPS receiver - it has no function any more - and sell it.