What you want is the iPOD cable with 510, 710 and 910 on the packet, I appreciate you have the 940 but trust me the iPOD cable from the x10 range will work fine with your 940 and the accessory mount (How can I be so sure - its exactly what I have fitted in my car) see
Here for one such advert - you won't find an iPOD cable for a 940 as the only iPOD cable ever produced was for the x10.
Not too sure about streaming the audio from the iPOD to the car deck via the 940's bluetooth - thats something I will have to try, but my initial thoughts are it won't work. Why not fit a large capacity memory card to the 940 and pu your music on that, the 940 will then stream audio via Bluetooth, your 840 can accept cards up to 32GB in capacity - Mike