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Looking at the old Sewell schematic, it shows part #76871 for the spoiler (you'll have to google the corresponding full part number), but nothing broken out for just the 3rd brake light. So, your only option is used, most likely. I think I still have a spare out in the garage, and I'm just south of you...
I'm going to give this place a try. Called and they told me usually the circuit board goes bad and if it is bad, it's $125 to replace and that includes return shipping.
They should be able to be purchased separately since that assembly has it's own part number: 81570-24050. To be clear, this is what you were asking about-- the red plastic assembly within the spoiler that encapsulates the LEDs. It's rather pricey though. Might be worth getting the LEDs replaced by a repair outfit. Actually, it might be something to ask Tanin Auto about since they are also familiar with cluster lighting repairs.
^ Good find on the electrical part numbers. It's tough to tell what exactly #81570-24050 is, though. One schematic shows it as the '97-00 style spoiler 3rd brake light, while another makes it look like it might be the parcel shelf 3rd brake light.
As it would turn out the led panel is good. Found this out after sending it to ledfix. Good to know they tested it and were up front with me. So I then decided to have a look at the harness. One of the two wires on the harness was broken. Reconnected it and everything works.
I followed the harness from the led panel down to the main harness in the trunk. It meets the main harness right next to the passenger side trunk arm. Pulled back some of the electrical tape and plastic covering and found it broken. Not really sure how it happen but I did go sometime without the trunk bumper stops in place and only recent replaced the with new ones. Could have been without the stops the trunk lid lifted higher, potentially stretching the harness further than it could go and eventually snapped it. My guess anyway.