On another forum I found this regarding the black screen of the mk4:

"The problem is a tiny in-line surge protector (like 20mA or something). It's inside a small, rectangular box in the cables that connect your Nav system. Basically, I had to cut the wires (two, IIRC) to remove the box and then used a multimeter to determine where the fault lie.
It should be inline with the blue and red/purple plugs shown in your picture. It's likely covered in cloth tape like the rest of the harness. It's less than 3/4" thick and about 2-3 long x 1" wide. But I don't REALLY recall the dimensions."

Can anyone tell me if if there really is that tiny "box" on the navi wires?