DS phat won't start after shell replacement

I have a problem with a Nintendo DS phat that someone gave me to repair. I replaced the shell of the DS and was carefull so nothing would brake. But after the replacement the DS does not turn on. When pressing the power button you can hear a short cracking noise through the speakers, but that's it: no screens flashing, no green light, nothing. When I plug in the charger, the orange light starts burning and does so untill the battery is fully charged.

I already checked the fuses with a multimeter, they are still intact. I also checked all the connectors multiple times. Anyone here who knows what possibly is wrong? I searched alot on google, but all I can find is broken fuses and bad connections.

Thanks!