HIGH cluster shows text messages like FASTEN SEAT BELTS, LIGHTS ON, LOW OIL PRESSURE, TIME XX:XX, CONS1 34MPG etc etc

LOW cluster doesn't show the text messages

A prerequisitie for fitting the nav is that you have the HIGH instrument cluster

If you have LOW, then you need to change the cluster from LOW to HIGH.
Only the HIGH cluster sends out the necessary data messages required by the nav for the nav system to work properly.