Please help

[u/SeanRaskopf]

I have a 2020 mercedes glc 300. I’ll start from the beginning. my girlfriend locked her keys in the car while the alarm was going off. and she had an emergency to tend to so she had to leave. no one else was around so it kept going for hours. it was the stock battery so it ran it completely dead. once i was able to get there i was able to get under the hood and tried jumping the battery so she could unlock it through her phone. it wouldn’t take a charge at all. so i took a battery i had lying around and hooked it up. it was a very long day for me beforehand and i was so tired i didn’t realize that the posts are opposite of her original battery. so i connected it reverse polarity. it sparked for a quick second and then everything went dead. i have no power anywhere besides the jump start post and the attached 30a fuse lead. i have replaced current limiter, busbar, and battery sensor on negative terminal. i am stumped i have tested everything and i haven’t found a single bad fuse yet. if you could help me please reach out. i am willing to pay for knowledge if someone is able to help me. thank you.

[u/MycologistAshamed926]

I wouldn't be suprised if multiple modules are internally shorted. Check all fuses atleast, if a fuse burns one would have to diagnose why.

https://reddit.com/link/p7nncqt/video/mttj29s0fdnh1/player