it might help if you could provide a photo, and I’m guessing as it is your first posting that you are still feeling your way around the forum, figuring out how to do it. Below the text entry box you will see in bold type “Attachments and other options.” If you click on that, various options will come up like taking a photo, or “Attach and choose a file” from your file of photographs on your computer.
As for the instrument location, I have instruments placed on both sides of the companionway entrance, where there is a way of leading the data cables up from the engine compartment below. In my case I mounted the instruments first on to plywood bases, then screwed the bases to the glass fibre mouldings, but with a soft gasket in between made from an offcut of vinyl floor covering But I’m guessing your instrument might be stuck on with some kind of sealant, and hopefully not a permanent adhesive. You could try using a long but thin bladed knife to see if you can get the knife between the instrument and the glass fibre and run it around the instrument in the hope of cutting through the sealant if that is what has been used. In your case from what you have said, the back of the instrument will protrude through a hole in the GRP so it is unlikely that you would cut any of the wires coming out of the back.