Fine and dandy, however we are talking a tail light – if it's a standard incandescent bulb it's pretty hard to screw it up in such a fashion that it's gonna kill the battery.
- If you run positive to positive and negative to negative, it'll work.
- Again, if incandescent, you can even hook it up backwards and it'll still work normally.
- If you don't hookup the ground or the positive properly it just won't work, but it still won't kill the battery...because there's no circuit completed and therefore it's not drawing any power. .
- If you short something out, it'll pop a fuse...not kill the battery.
Now, if it's LED the story is basically the same except most will not run on negative polarity, but it still wouldn't kill the battery.
The only plausible way the new taillight could kill the battery is if he somehow wired it to constant power, however as I mentioned one might assume he would have noticed the tail light was never going out, not to mention there wouldn't be any constant power circuit at that end of the bike anyways assuming he spliced into the stock tail light wiring.
This comes full circle to being a battery that suffered over the winter months because of being left in a probable low state of charge and then not maintained.
Ah haw. Exactly. There we go again assuming he would realize the light never went out and he has basic knowledge of assessment, cause and effect.
Maybe he left the key on for the entire install.
So many maybes.
To assume it is the battery right away is nieve.