A much neater solution is to remove the nav bracket completely, buy a separate zumo xt mount off ebay, they are 3d printed and seem to do the job. Buy the accessory bit of wiring loom and plug it into the existing power plug or cut off the power connector from your cradle. Lots of videos on YouTube showing how to do it.
The adaptor fits between the zumo and the nav mount without any modifications and is much easier to fit and no messing with wiring
https://www.zumouserforums.co.uk/viewtopic.php?t=1318