I connected it to a battery I had laying around in my garage. Basslink worked perfectly

I then directly connected it to the main battery. It keeps cutting out.

Voltometer says 14 coming from the main battery without the car being turned on. I burnt a fuse and tried again but still no results. I tried with car on and off.

Voltometer said 12 when I connected it to the outlets in the green box. No results.

The problem is coming from my battery since it worked perfectlty with the other battery I had in my garage (it is from a maxima).

Update: For the hell of it I added capacitors to the speaker wires. No change. Still cutting offf.

Buy a new battery?

Any suggestions what is wrong?