could be alot of things,take a volrt meter and with the key off test each fuse to see if its letting juice over a few volts thru it, will isolate the problem, hope that helps
The key being in the "on" position itself. As for rapid battery drain, you could have a bad battery or there could be a loose connector. Source: Personal experience
Dead cell in the battery itself which requires you replace the battery. Can also be any light on the vehicle that is on pulling power from the battery. It can also be a relay that is stuck closed.
something is causing a drain on the battery when switch is turned off
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Bad battery or a parasitic drain
Yes, that would definitely drain the battery.
If the alternator is installed incorrectly, yes, it can drain the battery.
If the defect is causing a parasitic drain, yes.
If the battery is worn out it may not hold a charge for 8 hours. There could also be a short somewhere causing the battery to drain down.
most likely somehting is not turning off such as an interior light that has a broken switch or you could have an electrical short somehwere.
Absolutely. If a diode shorts to ground it can drain a battery in a very short time.
you could have a short some where causing it to drain the battery. or the hatch or doors are not shut keeping the lights on in the car.