Electrical Weak power/no starting

So I have a starting problem. I use my mustang to deliver pizzas every once in a while for fun. I had been driving it for around 4 hours before this and it started perfectly fine everytime. Then it just wouldnt start. When I turn the key to turn just the electrical on, I have to wait like 30 seconds and then everything slowly comes on. But then when i try to start it, there is a single click and then everything stops. Now I also have an aftermarket radio, and just before the car wouldnt start it reset, the time changed back to the preset 12pm and stuff. That makes me think it was maybe the starter/radio wire or fuse. What do you guys think?

I was able to start it once by just putting a new battery in it, and doing an anti theft reset. But when I turned it off and back on again it wont work again, and doing the theft reset again doesnt work now.