You have what is commonlly called a Hi Joint, high restance open, in the low power electrical industry(communications). It is proablly cause by the bad joint heating up, when it gets hot enough it opens up and quits conducting, very hard to find when it works, easy to find when it doesn't work(open circuit) with a Volt Ohm meter, even using a test light can find them when they are not conducting Let us know what you find, BTW I had a Moroso battery cut off switch slowly go bad causing me to melt two of the battery terminals in my old Duster, what a mess Lots of gremlins out there Let us know what you find and do to correct it