If you can see anywhere near the top of the fixture can when you pull it down that inch...do you see a metal wire on each side opposite each other? Most of these fixtures are held in place by "spring wires" on each side or ears if you want to think of it that way. It could be that the fixture was twisted in circles during installation and that is "shortening" the wires leading to it and preventing you from pulling it down any further. Don't know what your fixture looks like, but when you say that voltage is 12.5vdc at the fixtures terminals...was this at the bulb contacts or what? I would expect to see 12vdc at one side of the bulb and ground at the other side...which would allow the bulb to light. If you measure 12vdc on both sides, then you don't have a ground connection. Ground should come into the fixture and go to one side of the bulb, while the other lead into the fixture should be 12vdc that goes to a switch, If it has one on the fixture) and then onto the bulb. Just some suggestions for you. Good Luck. John
2012 F350 6.7 L dually, 2013 3800RE with 6 pt leveling, Sumitomo 17.5" load range h tires, Samsung 18 cu ft residential fridge, 8k Morryde I.S. with disc brakes. Full timing since 2012.