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(1.) You need to temporarily remove the turn-signal assembly on that side. Those allan nuts are semi-tricky to access properly. It helps if you have allan wrenches with the ball-ends, as they don't have to be inserted at a pure 90-degree angle.
(2.) After taking off the turn-signal assembly, if you lie on the ground and look straight up under the passing lamp, you'll see the nut that needs to be tightened. The problem is, there is an electrical wire, passing through the center of that nut. So you can't just use a standard socket. You need one that has a slot cut along the side of the socket. Snap-on makes one like this, and probably other companies do as well, but I simply manufactured my own slotted-socket by cutting a notch into one with an angle-grinder.
(3.) After tightening the passing lamp with the slotted socket, you can reinstall the turn-signal assembly.
Hmm... this might have sounded more complicated than it actually is. The only real "trick" is that you need to have that special socket... whether you buy one (expensive!) or make one yourself (cheap!)
Here's a bit more info for you. I just looked it up in my service manual, and what I had referred to as a slotted socket (in my previous post) is actually called a flare nut socket. (Ya learn something almost every day!) Anyway, here's a sample of what an official flare nut socket would look like. See 1st photo, below.
As I was saying, you can easily make one one of these. It won't be as fancy as the Snap-on version, but it does the trick, and you'll rarely use it anyway. Just buy a cheap 9/16" deep socket and cut a notch into it with an angle-grinder or hacksaw. Here's what mine looked like. See 2nd photo, below. (Don't laugh too hard. My home-made flare nut socket may look crappy, but it functioned perfectly, and it cost me almost nothing!!!)
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