So I have an 03 G35 sedan runs beautifully but only problem is the radio. It works whenever it wants to. It will be working then out of no where I hear weird sounds and it just completely goes (the radio screen still be on) it seems to not be picking the signal up, people tell me to check the amp but can’t really find it. They say is in the trunk but it’s not really there. Also I’ve been starting to notice that when I let the car sit for a few hours or overnight when I turn the car on the radios starts working, then 2 to 3 mins later it just goes. I came to a dead end trying to figure out what’s wrong with it what’s should I do?

Well, your FSM is here:
https://www.nicoclub.com/service-man...F2003%2Fav.pdf
scroll down to "Click here to download a copy" for easier reading.

There's an antenna amp stuffed into one of your C-pillars. This book also shows how you can test the window antenna. Just some ideas. The "base" system was all one piece but the Bose had an amp in the trunk (and the word "Bose" on the cassette door).

This is what it tells me. Seems that I got 7 mechanical problems with the radio. Tried looking how to replace antenna window but there’s no video about it on a base radio, thank you for helping me I’m a step closer to find what’s wrong with it.

Hmm. It's nice of the manual to show us how to perform a self test but then it doesn't explain what the error codes actually mean. My guess is that a "mechanical" would refer to either the CD or cassette players as they are the only moving parts of the system. Have you got any media in those players? Is there a stuck CD or cassette in either player? Maybe the CD player thinks it has a disc and is taking over your audio? Just a wild guess.

I need to get a head unit that has the cassette deck in it. Still have tons of those things laying around.