Technical Radio head wiring

[Steady1234]

I have a 2014 fiat Ducato
And have fitted a cheap radio head unit , it’s in a Campervan, so I wanted to wire it through a switch so that it can also be switched on without the ignition on , I’m not an electrician but I have basic idea , so I thought to put a switched live feed to the ignition feed on the head unit ? But when I do this I am back feeding so all the ignition lights come on ?
Is there anyway of doing this correctly
Any advice appreciated

 

[NeilDD]

Simple way is to wire a fused feed directly from the battery
More complicated if using your current feed is to fit a diode in line (Think of it as a 1 way valve). The band on the diode would go to the radio end.