Battery seems to be drawn empty

Good evening in the round, I’m slowly turning something around. I built in a new battery, because the old one was apparently really broken and did not charge either with charger or after longer rides. Lima is fine, test-proof battery of my Brude jammed and it loads… Yesterday I packed in a new battery and measured it: 12.63 volts (approx. 18.00). Today against 12.30, 12.52 volts. Just measured again, 12.47 volts….I saw that it had been jammed now. ch Safety No. 15 (?) well for interior lighting etc. Current flows, despite standstill (so everything is off, completed, about an hour waited)… How can I narrow down the whole thing further? Result of all other fuses in the box = 0 mV. There is still the other fuse box in the engine compartment. Should I also measure here? And is there anywhere else to find a fuse box? V70 I, 1999, petrol, no special extras Thank you!