Changing the head unit on a Nissan Patrol GQ?

Hey there, I was wondering how I can change the head unit on my 1997 GQ Nissan Patrol RX. Looking at the radio now, I don't see how I can simply pyre out the current unit. I would get an adaptor if only the damn tuner was working. Anyway, long story short, how can I replace the head unit in my patrol and what things do I have to take into consideration when doing this?
 
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