As promised, here is a step by step guide on how to install a line-in cable to a B9 (without the RSE option installed).
Step 1: prepare the audio cable, in my case I used an RCA-3.5mm cable and cut the RCA jacks off. I twisted the grounds from both sides together.
Naturally you'll need 3 wires, a ground, a left and a right. You can start by stiffening up the wires by soldering some thicker wire to them.
Step 2: Pull the trim off the passenger side so you can acccess the screws for the radio/climate panel
You start on the right side and either pull (quite) hard on it, or by inserting a plastic gap opener into the side and pulling it off.
With the trim removed you'll see the first screw.
Step 4:
Pull the decorative cover off the gear changer, either by hand, or with a plastic wedge
Step 5: Pull the whole gear cover off:
You'll now see 2 more screws for the radio panel.
Step 6:
Pull the small piece of trim off to the right of the wheel, you'll need a wedge again (if you don't have a wedge try a flat screwdriver with some duct tape over the blade to prevent scratching your car
You will now see all 4 screws:
Step 7:
Disconnect the 2 connectors that lead to the radio/climate panel
Step 8:
Remove the 4 screws holding the radio in place: