Hi Eveline!
Did you tried a other/new AV cable? Would try this first and see if it works!
If that failed, then it might be the GPU. This won’t be easy to replace as some serious soldering skills and tools are required. It’s really difficult to find this part. So a donor board (with different issues) is probably required to get the GPU off and solder it in your current motherboard.
But, glady this is most of the time the AV cable. So please replace this one first!
Please let us know how it goes!