Ok after many wasted hours searching for a solution, I have found that the GTN switches on happily in the vanilla Carenado but not when REP is installed. I'm amazed that no-one found that out before.
[SOLUTION]
The problem lies in the default power source selected in the GTN settings. To make sure the GTN is able to be powered by the avionics master, go to Plugins/Reality XP GTN/GTN 1/Settings/Gauge Options:
Make sure power source is 'Avionics Bus'.
Bingo. All sorted.
Note you can also set to battery master, which also does what it says on the tin.
Sledge