Edit: After a little more research, this item is the "turn signal" that can be replaced ??

 

That bulb assembly can be replaced, but is quite pricey. It was a special design back in the earlier Acadias. Many sell for around $70 and sometimes difficult to find.
GM 13582912 - Google Search

 

Good find.
I replaced the entire assembly because it was leaking. The seal around the outside is terrible. I did attempt a regluing. I wasn't happy with the way it looked and spent the money on a replacement. I found the best deal on CarParts.com. It was $285 in 6/22.

 

yes, the round metal thing controls the stop and turn signal. Easy to replace.
While I have it out, I'm going to drill a 1/8" hole in the lowest part of the plastic assembly to ensure it never accumulates water again. APPARRANTLY, the metal housing also generates a little heat to help with the moisture problem - this tells me that GMC is well aware of the problem and this was their "Cheap Fix". (????)

 

Your fix of the small hole is probably a better fix than the heat. The condensation needs someplace to go. I have done that on several tail lights in the past. I am not sure if any taillights seal very well anymore. If you look at other cars on the road or in parking lots, you can see quite a few with water droplets in the lights. I usually drill a 1/8 hole in the bottom corner from the outside.

 

If the assembly gets moisture into it, that usually just means the seal around the housing has been compromised.

The spiky fins on the back of that turn signal light real purpose is to act as a heat sink for the LED light itself.
Most, if not all, large LED bulbs need a heat sink to radiate the heat away from the electronics in the bulb assembly unless it is a very small LED light like those used in interior dome lights, map light, dash lights, etc.

 

Update: I sealed the top and sides of the plexi housing with silicone while awaiting new parts.
Which was a waste of time since I ordered a new housing AND the stop/turn signal element which will be here "soon". I opted for the new housing as the water in the plexi case tarnished the silver reflective finish, which will only deteriorate further over time. And when I trade it in, I don't want that to be an issue.
Depending on the seller's actual location and availability.
ISSUE: I am very near Orlando, FL where the "Shipped From" is stated on the Ebay location. I "Contacted Seller" to see if they offered in person sales . . . no answer. 2nd Question = do they "actually ship" from Orlando or Drop-Ship from NY, Calif, or China ?? Robo-Reply = all the information is in the listing. 3rd reply = is the same Robo-Reply. ..... estimated to be here Friday or Saturday. I'll update after the freakin smoke clears.
Stay tuned

 

That's funny. I did the same thing. I have the old, sealed housing on a shelf in the garage, "in case I need it one day."

 

Metal spicky thing is a heat sink.
Probably for little tight space, so they design it that way to increase surface area. More surface area, more heat dissipation.

 

Welp - one more FIASCO to add to my frustrations - - - the ebay listing said it was shipped from Orlando, FL (which is where I am). It was actually shipped from New Mexico. EVERY inquiry about the status was answered by the infamous "RoBo Response" - never a human being. One reply was a RoBo reply from "Frank" at Parts Geek, LLC. and he (it) never answered any further inquiries.
The part NEVER arrived. I filed the "Did not receive the order" with Ebay and got a prompt refund. The PartsGeek supplied Proof of Delivery from FedEx . . . a person named Jose Fernando signed for it and the box weighed 49 pounds . . . So I have no idea what PartsGeek actually shipped.
Anywhoos, With a little more googling, I found one at my local O'Reilly's for $5 more and their warehouse is a few miles down the road from me and the special order was delivered in less than 5 hours after the order.
Other than the ebay frustration, the parts were a perfect Plug and Play replacement and I'm a happy camper once again.
Hope ya'll have a happy and Blessed Holiday Season. (and problem free).
Thanks to all for following along and your responses.
John