pikpilot

I use this which may be similar: https://www.amazon.co.uk/Gummi-Pflege-Rubber-Stick-100ml/dp/B007TNOQDI
It has the advantage that you can put it on all the outside rubber to protect it from UV in sunlight. Just try to avoid getting it on the paintwork or you spend just as much time getting it off the paint!

With the radio on, press the Set Up button the right, then on the screen press Media, Aux Input Level, and then select High or Low

In your first photo it looks like all the brown wires (neg return) have been joined together in a screwit or crimp connection to a single brown wire taken to the dedicated ground stud on the body.
In the third photo you are looking at the fuel flap release solenoid. Note how easy it is to replace by just loosening the screws.
For the 12V feed to the trailer you need to run a 20 A cable from the fuse box at the drivers end of the dash. It needs to be this size to run a fridge etc etc.

Also if you press hard on the brake pedal whilst pulling on the handbrake you can add even more force. I once drove my car (VW Passat) to Heathrow for a flight one day and my secretary was to drive the car back as I would be away for some time. Unfortunately I had used this technique as the handbrake was not too good. When she tried to drive off she could not release the handbrake. Several people tried apparently until one man knew that you had to press the footbrake again whilst releasing the handbrake.Last year the handbrake on my mk1 Octavia slowly became less effective to the point where it would not hold on the steep ramp at a local recycling centre. Local Skoda dealer found both rear calipers sticking. One was cleaned up but the other had to be replaced.
Perhaps this is your incipient problem.