Hi Rusty, welcome to the site!
We're more of a Ford truck site, and this forum is specifically for the 4-cylinder engines so drivetrain questions don't usually fly in here. That said, lot's of folks here have Mustangs as well. I can't say as I'm familiar with the rearend used in the the 4-cylinder 'Stang, but first I'd get some fresh batteries in that flashlight and have another go at spotting the little bugger you're looking for. You might try a little pocket or inspection mirror as well, you might not be able to see the adjuster because you can't get your head up in there at the right angle. Shine the light in the hole and hold the mirror up at an angle and move it around to try to get a view of the thing. I've also shone the light right into the mirror and used it to project the beam of light into the hole.
You might also try whacking the outside of the
brake drums with a hammer to try and break the
brakes loose. (Be careful, cast iron cracks...!)
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