There are numerous guides here for installing aux in. You need a cable from eBay or other retailer specific for your car. This will be around £5. In my case, my stereo was a Grundig GHS5100. Although I picked what I thought was the right cable, it turned out it didn't work. Some googling and I found the below pinout diagram so I moved the pins and the aux in started working.
1 Aux L
2 Aux R
That's ok if you just want aux but if you want bluetooth then read on. First you'll need to order a bluetooth 3.5mm adapter. I suggest one like this
The adaptor takes a bluetooth audio signal and transfers it to 3.5mm jack socket. By connecting the aux cable to the bluetooth you can stream music wirelessly. It is powered by USB so you just need to provide a USB power point like a 12v cigarette lighter adaptor.
I decided I wanted a stealth/factory install and to keep my 12v socket free for other devices. Here's how to do that.
Remove the gear stick gaiter and unplug the 12v plug. We are going to tap into this as it is a switched live and will cycle the bluetooth module everytime you start the car.
You don't need to test the wires for live and earth as I did it but if you want to and don't have a multimeter you can make your own, like I did
Using wire strippers, strip back a small section of sheath on the black wire (ground) and solder a wire from this wire to the metal tab on the collar of a cigarette lighter USB charger. Insulate this with heat shrink or electrical tape. Now, ensuring the ignition is off, do the same with the red/white wire and solder it to the tip of the cigarette lighter USB charger and again insulate it all.
At this point you can test the charger.
Route the bluetooth USB cable down the back of the stereo and into the centre console. Plug the USB cable into the USB charger and refit the gear lever gaiter.
Plug the USB cable into the bluetooth module along with the 3.5mm aux lead from the back of the stereo. Turn on the ignition and it should light up.
For security you should tape the connections to stop them moving about. Feed it all behind the stereo and refit the stereo.
Enjoy beautifully streamed music and hands free mobile.