How do you change the battery on bmw 645 ci - 2004 BMW 6 Series

https://youtu.be/yg4vYPTU668
there are three main locations for BMW trunk, glovebox, under hood, here is a vid of one issue.

DOT Brake fluid is fine. Just go to any parts store and ask for it. The brake fluid reservoir will be under the hood, drivers side against the fire wall. Small clear container with black cap

On this website you find fuse box diagram and description for BMW 6 - series
BMW 6 Series E63 E64 2004 2010 fuse box diagram Auto Genius

It sounds like there is a faulty sensor, as your trunk is not opening without the key I would suggest its your trunk/boot sensor.
As a safety feature the cars roof will not move if the trunk/boot is open.

If it has other volume controls in the back for passengers then you may have to turn the volume up from there.

apart from a diagnostic test which before pulling out any more money would get thsi done it could be a few things faulty plugs coils or worst case ecu which is where the spark gets sent from or regulated from but get it checked first as it should show up on the test what it is

you need to replace the wear sensor it will rest itself

Rear brake Change light was on at 600 miles to service .got brakes changed and tried to reset dash on rear brake but went to. -15000 then brake light came on instead how do I get the light off?

There should be a black cover behind the glove box. Remove the cover and look on the back for which position it is in.

If you have the diamond shape key, the battery is not replacable. If you have the square shaped key, the battery is replacable, but that is obvious because of the screws on the back of the remote.if you have the diamond shape key, it is recharged when its inserted into the ignition. If the key hasn't failed, taking a long trip 60 min. or longer should recharge the battery. If there is no change in the operation of the key after your long trip, most likely the key has failed and needs replacement.