I saturated my M365 tail light with water while cleaning the scooter and it has become dim and barely visible. Amazon or Aliexpress has plenty of cheap replacement tail lights. However, the cabling and housing is almost certainly just fine. It's only the LED that has been damaged and needs replacing. And ordering anew means missing out on some fun DIY.
I always thought the light too low-profile, even when it was working properly. Of course, this is the perfect excuse to make some improvements. I added another LED to the assembly and resealed the housing to prevent further water intrusion.
The first step is removing the fender. There are three rubber covers over the screws that are pretty easily removed with a small flathead screwdriver. Be gentle so you don't cut into them too much.
Once the screws are out, it's only still fastened via the brake light connection. Use a pair of needle nose pliers with the scooter flipped over to gently squeeze the plug and work the fender loose. You'll likely want to brush off excess dirt at this point as any amount of prior riding will build up lots of it.
Completely remove the cable starting at the plug. Note there is a dark mount holding the white plug. Carefully separate that so you are able to begin freeing the cable. Part of my cable was also glued under one of the holders. I sliced it out with a small flathead screwdriver.
With the cable free and the light housing out, applying some force at the black/red seam pretty easily opened the unit. Be more careful than me so that you don't scrape some skin off of your knuckle like I did.
At this point you should see any trapped water start falling out. The white part holding the LED comes off of the transparent red lens with a little prying. Desolder and discard this to allow new LEDs to shine out.
I repurposed some flat red LEDs from another scrap lighting board I had around. Any SMD (surface mounted) LED will work, though I would avoid high-intensity. While I believe the scooter can supply up to 500 mA to this light, the existing one definitely isn't even close to that. It's probably more like 20 mA. I may revisit this -- playing it safe for now.
I soldered two of these SMD LEDs slightly spaced out and facing directly outward rather than backward-reflected like the stock design. Nothing fancy for securing them within the housing— there's not much room for them to move around. The soldered connections hold them well.
In prime hacking style, I sealed the two halves up with electrical tape and dropped some superglue around the wire going into it. I think that should work fine, but we'll find out when next winter rolls around. I hope I saved you waiting eons for the whole replacement assembly.