Recently bought a 2005 G35 coupe because I got a good deal on it. The stereo and the climate control were not working so I got a used unit to replace mine. Looked at DIY videos and installed the used deck into the car today. Turns out the new one doesn't work either. The AC was always on even after I installed the used deck. I disconnected the battery and now it has turned off. Does anyone know what the problem might be? Was there even a problem with the original deck? Both of these decks don't work. Is there a problem with a fuse?

If anyone knows what's wrong please help

 

Well, it's a very common failure of the factory radios. Any replacement is going to be as old as the one you're replacing, so it's possible they're both failed units.

The a/c always on, however, may also be caused by a disconnected cabin temperature sensor, see this thread here for more details:
https://g35driver.com/forums/audio-video-electronics/227673-air-conditioning-issue-after-double-din-install.html