This is not an original accessory to my Thunderbird, and therefore my shop manual doesn't tell me how to install it. Installation instruction for this part?

Need a lot more information than that. The part number is not recognizable, at least by me. Who is the manufacturer? The standard shop maunal does not cover many portions of the body structure so it is not unusual to find topics that are not addressed. You may need to refer to a "Body and Interior Assembly Manual" which is a separate book. What year is your 'bird?

As far as I can tell it should be a drain trough for a 65 or 66 ragtop. I would guess lt and rt. What car are you working with?

I'm not sure exactly how the mount, but they go in the trunk under where the trunk lid and package tray/flip lid weather strips meet. The direct the water into the rear fender well. There are drain hole in the bottom of the fender well so the water can drain out. I was told by a T-bird supplier that most people don't install them because they wind up rusting out the fenders. I hope this helps.

Thank you

1964 Conv.
Parts from TBHQ
Part#:B45118/9R