Hello Koxi, yellow and red together on + (max. 15V) black and handbrake cable on ground (-) check the mute cable, not that it is accidentally on ground! You can use the red one via a switch to simulate the ignition if you want!

Which speakers do you have connected? Car radio (4 Ohm and max. 50 Watt) check if the speakers are working.

If there is still no sound, then you have burnt out the output stages of your device 😬

Is it possible that the speakers themselves still need power? Otherwise it will be difficult, as far as I know the unit itself cannot control different audio outputs.
Check the cabling again for sure.