One of the best mods I made to my boat was to fit a PIR activated light, it detects as soon as I enter the companionway. I made an LED board for it using discrete LEDs, but it wasn't very bright. I've just upgraded it with four G4 LEDs mounted in sockets on a custom PCB:
(http://i77.photobucket.com/albums/j54/nigelmercier/Dolcetto%20NEW/LED-PIR-003_zps83tpfoiq.png~original)
The switch selects between Auto/Off/On
The basic unit is here: http://www.amazon.co.uk/exec/obidos/ASIN/B0040H975S/b0404-21 (http://www.amazon.co.uk/exec/obidos/ASIN/B0040H975S/b0404-21)
Cracking Idea Nigel!!!!!!!! Gues what I will be fitting the weekend! :tbu
PIR lights are incredibly useful. But rather than have the problem of wiring them in to the 12v circuit, I've fitted a number of battery-powered PIR lights, in the fridge, in a cockpit locker, and in the cupboard under the heads sink where all the seacocks, etc, live. I got them from Banggood (http://www.banggood.com/buy/SKU184604.html) and they cost about 4 quid each. As soon as they detect movement, they light up. And they are light sensitive too, so they only work at night.