Hey Jamie, you checked all the fuse? i just replace my died battery today on my R33 Gtst with stereo sound system. and then after that i start my car then the battery light is on on my dash and no power on the radio as well. Pretty much same what you having right now. but have power on window/light/Air cond etc. And then i went back to SuperCheap because i got the battery there, the guy checked for me and my power was on 10.5v on starting engine and V doesnt go up when we rev the engine. And then he said to me, the alternator is broken and need to get it replace as soon as possible otherwise it won't charge the battery and the battery will go flat very fast. I am not 100% sure he is right but i will go around and ask about the alternator.