I'll be adding to this whenever something interesting or new is done on the bike, hopefully remembering to take photos as I go. For new and prospective owners, this will be a reference for what they'll probably have to do themselves. For me and for everyone else, it'll be because my dumb arse...
hondatrail125.com
I installed mine here. It's light on details, but the TL,DR is I ran the SAE cable from the battery and followed the snorkel up, so it exits right at the rear of the airbox, which was part of a plan to power a rear case (that ended up not being used too much). Hardest part was not dropping the screws into the bowels of the bike. I can take more detailed photos if you need them. It does have an inline fuse so I do have to pay attention to how many amps are flowing.
The other popular place to put it is in the space above the engine. One advantage of that location is my cable has had a decent amount of sun exposure, so I'll probably have to replace it every 2 years. However given the difficulty level of installing it, I'm pretty ok with that - it'll probably take about 2-3 minutes to swap, and I'll check the battery health at the same time.
EDIT: I used a 2 ft cable that I had to trim slightly; I think 65cm-70cm (25" to 27") would be pretty optimal. Longer and it'd have too much hanging out, shorter and it wouldn't clear the airbox. IIRC I used
this but had to cut the plugs because they were too large to fit under the battery screw.
