Hello Marco,
You are probably aware that the the speakers outputs are Bridge-tied
. It means that none of the two connections are grounded, they are just two opposite polarities of the Audio signal.
So if you want to use a jack for output you'd better use one stereo
jack for the right channel
and one separate stereo
jack for the left channel.
Then, for each of the two channels, the wiring (to provide the signal to the speaker) should respect the differential mode, one polarity connected on what is usually the left pin, the other polarity connected to what is generally the right channel, and then ground is ground or it is left
unconnected
I hope it sounds
clear