Installed radio now battery seems to be dead!

So I installed a new radio system in my car and it was going well, I’m not experienced or anything but I had to hardwire it, which was simple as I had the wiring diagrams so it was just a game of matching, when I finished I proceeded to to the car on but not start it and it worked, everything seemed fine so I left it on while I connected the back-up cam, nav, and mic. When I finished I decided to start the car, that’s when it went downhill, it would cark and turn over but that’s it, I checked the fuses and wiring of the radio and it seemed fine, I tried turning the car again but this time I only got a clicking noise, then all my lights started flashing so I turned the car off waited a bit turned the car on but now all the lights were dim. I’m sure its not the battery as it was only a month old but when I checked the voltage of the battery with the lights on it read a low 4v. What caused this. Please help!

If the door was left open for an extended amount of time while performing the work,the dome light must have drained the battery. Recharge the battery and it will be fine

I’ll try that. But I forgot to mention that the battery was disconnected for most for most of the installation process.

Recheck the connection