Power drain issues

Well, to make a long story short. My aftermarket stereo system in my car has a capacitor, while the car is off (even overnight) It's reading about 13 - 13.2 volts .. Something is draining my battery and I can't get the car started after leaving it off for a few hours without jump starting it.. The most RECENT changes to my vehicle before this problem started occuring were:

Voltage regulator got knocked out and had to put it back in, the retention clip is broken.

Changed the head-unit in my car, was professionally installed by a company called Short Circuit, I think they're local.

AFTER the problem had gotten to the point where if I tried to start the vehicle, everything electrical would die in the car until I slapped jumper cables on it, I replaced the battery terminals. I removed the negative terminal grounding separator plate due to rust, and used one that keeps wires separate that I bought from AutoZone....

The fusible link crumbled and I had to wire in a new one... Nothing is touching that shouldn't be to this, I've made sure of it. This is a link I got from autozone with a 20amp glass fuse, I've spliced both ends of the factory wires in to either end of the unit...

Any thoughts would be great.

First I would have the battery tested. The 13volts your are seeing from the aftermarket is just for memory presets, etc. I would then test the alternator output, 1st with no accessories on then everything on. lights, blower motor, etc. If all that passes, then voltmeter placed in series with the positive cable and battery. Measure amps. Anything more than a couple milliamps is suspect. All to then is start pulling fuses until the amperage drops and you found the culprit.

the battery is definitely good, I was talking with Vett on the AF Chat, I am going to buy a circuit tester and start pulling fuses / relays tomorrow..

I know that the voltage regulator in the alternator keeps coming loose because the retention clip is missing that keeps it in the socket... but if that's loose or disconnected, it shouldn't drain my battery right?

Digital volt meters are cheap, under $15. Unless the lose regulator isn't charging the battery, I don't think it would discharge the battery.