Right, this is from my head ..... Matt black insulating tape .... perfectly matches the texture of the knobs. Clear "sellotape" . A set of children's coloured felt - tip pens. Start with the insulating tape. With a very sharp "Stanley" knife or surgical scalpel and using a steel, straight-edge as a guide, carefully cut your "+" and "-" out of the tape (or anything else, for that matter) Let the artist in you, shine!. Now, stick a small piece of "sellotape" on the knob(s) to coincide with the symbols. Chose a children's felt-tip pen that corresponds to the red illumination of the Audi instrument display. Experiment .... if it isn't dark enough, put another "coat" on! Now apply your freshly made covering to the knobs, making sure that they are trimmed exactly to size. As I said, this is from my head, not from practice, but just give it a go ... It can't do any harm!
David