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Among other things i fix automotive led clusters for a living, proper way to fix is to cut from behind. I use dental lab motor for this, it is more melt than cut, dremel with 1.5mm drill bit works just as well, dril a small hole and use drill bit to cut sideways, be carefull not to go too deep, cut 2-3mm INSIDE of a edge that holds foam gasket (slowly remove gasket before cutting) so you dont need to seal it for waterproofing again. Solder joints on those leds dont usualy go bad unless seal is broken and water finds its way to circuit board, leds fail internaly from vibrations, those are silicone sealed led smd chips. some leds start working after reflow cose you move led internals when reflowing but it is a short term fix, most of them will fail again except if you are extreamly lucky. I dont fix water damaged tail lights cose they fail again and again. To identify failed leds after you remove circuit board attach 9 to 11v 1A power supply (so you don't burn any leds during diagnostic) touch them lightly on the center with tweezers, if this does not work try shorting out one at the time until the rest of the led cluster turn on (e90 tails have very stupid electronics, if one led fails whole cluster stops working or start flickering). Replace any suspicious leds, they are dirt cheap. When you replace all failed leds test everything on a 12v power supply then put everything back together and use flat tip solder iron to weld plastic back together. Not super easy but it can be done under 40 min per light, it will stay sealed and repair will not be visible until light cluster is removed and if it fails again fix can be redone, often not the case if you removed and glued lens. There you go😀
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