The other problem with the battery drain.
You need to figure out where the current is going, normally with the key off the kill switch is dead but a previous owner may have made "other arrangements"
Remove all the wires off the battery negative, group them together and connect a small lamp between this group and the battery negative.
When you turn the key on the lamp will operate at full brightness, when you turn the key off it should go out, right?
If it doesn't go out then by a process of elimination find out where the current is going.
A word about the small lamp.
Even the tiny idiot lamps draw about 50 milliamps to glow so quite a significant drain would go un-noticed. A 12 Volt LED on the other hand only requires a few milliamps to light quite brightly i.e. it's much more sensitive, if the LED doesn't light you are ok. (just make sure it does go when you turn the key on)
In this case you would wire the LED with the positive wire on the chassis.
Note: I didn't suggest reading the leakage current with a multi-meter, there's a good reason for that.