I got annoyed at the ridiculous way that AUX in is added to the older RCD300. Here is a solution but it does require hobbyist soldering skills. USE AT YOUR OWN RISK! That said, it is very easy and safe for someone with some technical ability. This mod disrupts the signal line from the AM/FM radios when plugged in and injects the AUX out from your device. Decoupling is not necessary as the RCD300 already has decoupling built in. This is a standard line input, not digital.

I installed in the fascia of the RCD300. However, you could run a shielded four core cable to a remote location. In either case, a switched 3.5mm stereo socket is required.
Steps:
Open the top and bottom covers. Remove the CD screws and unplug the ribbon cable.
If you are installing in the fascia then remove the fascia and disconnect the ribbon cables. There is room under the buttons for the socket.
Locate the connection points.


Locate the connecting tracks on the underside and cut them.

Clean the solder mask from the pads around the holes on the topside to expose the copper.
Solder connecting wires to these points. Solder your earth wire to the metal can(shield) of the radio.

Reassemble and test.
Volume and tone controls work as normal. Radio must be selected first for this to work.
This has been a success on various makes of vehicles. Make sure that the radio is reconnected when you unplug the stereo jack plug. If it does not work then check switch configuration as it is very easy to connect in reverse.
Enjoy
I installed in the fascia of the RCD300. However, you could run a shielded four core cable to a remote location. In either case, a switched 3.5mm stereo socket is required.
Steps:
Open the top and bottom covers. Remove the CD screws and unplug the ribbon cable.
If you are installing in the fascia then remove the fascia and disconnect the ribbon cables. There is room under the buttons for the socket.
Locate the connection points.
Locate the connecting tracks on the underside and cut them.
Clean the solder mask from the pads around the holes on the topside to expose the copper.
Solder connecting wires to these points. Solder your earth wire to the metal can(shield) of the radio.
Reassemble and test.
Volume and tone controls work as normal. Radio must be selected first for this to work.
This has been a success on various makes of vehicles. Make sure that the radio is reconnected when you unplug the stereo jack plug. If it does not work then check switch configuration as it is very easy to connect in reverse.
Enjoy
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