That's actually the second time I've seen someone mention using regular off-the-shelf 3157 style LED's in the middle position on a GMT400 and not having a problem. SRCK as mentioned do exist and are for this application. It makes me wonder if some of the newer bulbs are designed differently so that they at least don't go up in smoke like earlier designs would.
Regardless I would be buying SRCK's for the middle socket because that's what it takes.
Here's what you won't like hearing. MOST LED bulb replacements are NOT brighter than the stock incandescent bulbs - especially in direct sunlight!
You really have to spend some bucks and look hard for truly bright LED replacements. So far the brightest I've got were from Autolumination, the projector-style "80 watt" (keep in mind that's still just advertising hype) but they are definitely bright.
The other problem is they don't make the same light pattern an incandescent bulb does so the reflector in the housing isn't doing the same job of filling out the lens with light.
Years ago these were about the brightest you could get from V-LED's but they don't offer them anymore. Brighter stuff has replaced them. I dunno if I like that Triton stuff they sell now, though.
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These are the projector style from Autolumination, definitely brighter.
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Also you can make a super-easy hack to your stock flasher module to fix the hyperflash and not even have to buy a new flasher. It comes down to just carefully prying the cover off and cutting the leg off one chip in there. Hey if you kill it, you can just go ahead and buy that LED flasher...but I'm telling you it's dead easy and it works.
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Richard