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Under the seat would be another decent place for aftermarket radio if want keep dash standard.
Got offered some doof doof gear for mine then got reminded it's too angry & loud anyway.
Look forward to colour going on!
For audio could always put in one of these
Alternatively as you use your phone for music use the standard head unit and have it modified for a 3.5mm aux input. Add an amp and decent speakers under existing covers. Not high end, but effective.
Have a look here discusses both 3.5 jack and bluetoothDownfall to that is that I'm picky when it comes to cables. Must be blutooth and I'm also going to try and build a remote into the steering wheel. I reckon I'll either have to go with one of those expensive retrsound units or install a current head unit in the boot.
Thanks for the suggestion though. The more advice I get the better. Interesting that the original units can have a 3.5mm jack added to them.