Hi!
I advise to clean the AV port first. If that didn’t worked, then replace the AV cable for a new one. If that didn’t worked, then try the console on a different TV to determine if there is something wrong with the console. If the image is like still super dark, then open up the console and clean it all (also the cartridge pins on the motherboard) and try again.
Could you also share some pictures regarding how the console is connected to your TV?
Please let us know how it goes!