Retrofit speakers how?

I bought a second-hand Seat Leon and recently installed a new radio. Unfortunately, the speakers are not very good. In the trunk flap speakers have already been installed, as there are corresponding recesses there. Now I would also install some there, but I am not sure which speakers I need exactly. In the trunk itself there are several cables, as can be seen on the photos. In addition, there are two audio cables. in the dashboard, where the radio is connected. Does anyone know what the other cables are for and whether I need an amplifier in addition? I’m not looking for anything special, but just reasonable speakers for some music in the car. Thanks in advance!