Dim LED third brake light ('13 Impreza hatch)

We had the third brake light (high mount stop light) replaced under warranty right at 60,000 miles because at least one of the individual leds was dimmer or not working at all, and the dealership said it wouldn't pass state inspection. Paperwork said it would have been $241. Several years later, and the replacement started having the same problem. Now, it's extremely dim altogether, with one or two completely dead leds. I know I can get a replacement for around $100 plus shipping, and it takes 10 minutes to replace the whole thing, but I'm wondering if anyone has actually diagnosed this or found a fix. I've seen a forum post about Forester (I believe) third brake lights dimming, and the culprit being a 10 cent resistor or capacitor. I don't think the board on the Impreza/Crosstrek light is accessible without some heating or cutting involved.

What's weird is that every time I take the light out to try to diagnose it, it seems to be bright again for a while afterwards (but still has some dead leds). I tried wiggling the wiring/boot and couldn't make a difference, tried plugging and unplugging numerous times to clean corroded connections, nothing has an immediate effect that I can see.