How to make your car radio play when key is turned off?

hi so i was wondering how to make your car radio play when the key is turned off? its first time when i hawe this problem with bmw e34 520 iv had e34 520,e34 525,e34 518 and non of them had the same problem i push the button to turn the radio on and it start playng but with this car i just cant make it play... show more hi so i was wondering how to make your car radio play when the key is turned off?
its first time when i hawe this problem with bmw e34 520 iv had e34 520,e34 525,e34 518 and non of them had the same problem i push the button to turn the radio on and it start playng but with this car i just cant make it play when the car key is switch off i would appriciate some help

thnx in front
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