OK, I have check the 3rd light with 12 volts and it comes on fine so obviously there is a loose connection somewhere, are there any obvious places. Also does the 3rd lamp wiring come straight off one of the back brake lights because if so that would be a short route to check. Night time here now and getting dark so will probably carry on checking in the morning so any suggestions would be great. Thanks.

Just moved this from Electronics Thread as realised I may have put it originally in the wrong place. Sorry.