Issues with battery drain and draw

[Vincenze]

You should disconnect the negative connector from the battery, set a multimeter to the 10A setting and connect the wires to its ground and 10A connectors. Close the doors and take out the light bulb from the trunk.
Then, measure the amperage between the battery and the negative connector.
It should be close to 0.035A.

You can set the multimeter to measure mV, and then measure the voltages between the two sides at the top of each fuse. Only the fuse #21 should have small voltage.

[ThomasE30]

So my e30 is modified and the electronics and everything is a bit of a shit show to say the least (rn the radio wiring is just wrapped in electrical tape and sound is coming out of only the driver speaker which was lazily put in door card, and volume will go in and put over bumps, needless to say I hate wiring). When I bought the car the PO installed a dual USB port that is connected to constant battery power at three little switches for “radio, main fuel and power, and then accessory, like the 3 different key positions, as well as a push to start button next to them where the factory cig lighter would be. If I don’t run the car once a day the battery dies and can’t start the car. I just recently replaced it with a brand new 650CCA from autozone. Could this be because there is a battery drain somewhere with all the electronics, or could it be the battery doesn’t have enough CCAs to power all the annoying useless electronics I need to remove? Right now there is a Amp in the back connected to the battery and then some shitty speakers and just lazily done audio wiring which I planned to take to an audio person to wire in my new JBL sub, and Polk speakers I got for free. Thanks for any help. The stupid push button start and switches will also be removed soon I just need to find the time to look at all the wiring under the console and how to wire it back so the key ignition starts the car. The PO is a very kinda guy and has already given me some of the old interior pieces that he had and would have no problem helping me out with those so hopefully that can be out of there soon.