It's been a while.... I'm back!

[87LX2.3]

You guessed it, my sad little Mustang won't run again. This time I'm sure it's the battery. It all started about 3 weeks ago, I got it running where it would hold a charge for days. Then I put in a new radio and it held a charge great for a couple days. I started it up one day and it took a bit for it to start but it did. I came back that evening and it had died. I was left stranded at work untill someone came to give me a jumpstart. I pulled the radio fuse and recharged it, and now it won't start at all. What do I need to do? Thank you for all your continued advice toward my rotten little car.