Why is the raio/cd not working?
 

I just purchased my first car, a used 2005 Mazda 3 sp23. We had to leave it at the dealer for a few days so they could repair a dent above the left wheel. After a few days of waiting and the dealer telling us to keep waiting, they finally delivered it to my house. They said the battery had to be replaced so they put a new one in. So I turn the car on and everything works except the radio. It keeps flashing and no audio plays and no text appears. I tried turning the car off a few times, still nothing. I tried a CD, didn't go in. I'm absolutely PISSED. We've had a lot of trouble with this dealer and now this just puts the cherry on top. Please, if you know anything, let me know. Thanks in advance.

*the radio was working perfectly fine before we left the dealership.

maybe some wires are loose in the back its very easy to get to it u want to check just let me know i can walk you through it

also maybe it is a fuse

Yes, please walk me through it. Anything if it may help.

here i found a video some ppl r better at seeing how it is done. and saves me time typing everything out lol but it is really easy.

Thank you very much. I'll have to try it when I get home.

Yup, definitely check the wiring behind the HU (unplug and replug even if it looks fine), though why that would become loose when replacing the battery is a good question.

Check all the related fuses both under the dashboad at the PJB and in the engine bay. If you have a multimeter check them with that as sometimes a visual check won't reveal a dead fuse.

Oh yeah, and make sure all of the plugs on the PJB are fully connected. Again fully disengage each one and reconnect it.

I just figured out that the radio/CD/clock only screws up when the A/C is turned on. So what could be causing that?

sounds like electrical gremlins in an old vehicle... Follow ICSPOTS recommendation of checking the PJB - and look for corrosion on the connectors there while reseating them.
I'd tell them to fix it or just trade it back and find something (and somewhere) else. I'd assume that if they messed something up, you are just finding what died first and it may be a long process of problem after problem (since you don't get along with the dealer anyway).
Good luck