Just saw an articel on thw txboard:
https://tx-board.de/threads/radio-12v-zuleitung-auf-zweite-batterie-umpinnen.113324/
Unfortunately in German and to see the pictures you have to be (free) member.
Maybe worth reading (author bazzman):
„Last weekend, I rewound my radio to the second battery under the driver's seat.
First, I uncovered the prepared safety rail under the driver's seat box. The supply line to the empty safety rail is connected under a lot of adhesive tape from the factory with a line to the main battery. After removing the tape, you can disconnect the cable well and attach directly to the second battery (do not forget to switch main fuse in between). For the original fuse holder, I had made suitable inserts "Contacts plug 000 979 227 E (4.8mm)", e.g. at Ebay. So you can then hang your own consumer in the safety rail:
IMG_5629.JPG IMG_5630.JPG
Now we went to supply the supply line to the radio with power from the second battery. To do this, remove the fuse holder B and C under the dashboard (loosen all the screws of the panels, then loosen the four screws in the fuse holder and thread the fuse holder backwards out of the panel:
IMG_5649.JPG IMG_5650.JPG
The radio was on fuse SB 33. In order to be able to remove the cable from the fuse holder I used a Auspin tool, e.g. Amazon. Before spinning out, the lock (violet) must be pushed up by approx. 1mm to release the pins. With the spin-out tool was spinning then super easy and done in 2min. The pins for the fuse holder are the same as those above for the safety rail under the driver's seat.
IMG_5651.JPG IMG_5652.JPG IMG_5660.JPG
Then connect the cable from the fuse holder in the middle under the floor through to the driver's seat box and via 30A fuse into the fuse holder.
During the construction I have also been able to take valuable hints...“