I'm having electrical problems with my car and would love help troubleshooting.

In my 2001 Toyota Echo my break lights,radio,console lights,fan, and wipers won't work. My speedometer and fuel gauge sits at 0. My check engine light was on then turned off and my scan tool wont turn on. Its stuck in park. There was a lot of deposit on my battery terminals (a lot more on the anode) and the fusible link seemed to be blown so it was replaced but after it was replaced it refused to turn on until I cleaned the terminals. when my car is running the battery usually puts out 12.5ish V then when its on it puts out 11.5 and seems to fluctuate from 12.8ish to very low. It all started when i pulled into a parking lot and i felt a slight power drop then saw my speedometer was at 0. When I came back out to my car i turned it on played a song on my radio then after a minute of it playing my radio turned off. I was stuck in park and had to manually override it to put it in drive.
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