There are many options the first does it have round pins or square pins this can be found on the lead attached to the ISO conversion lead,then there are radios with inbuilt cassette,with inbuilt cd and inbuilt cd mp3,satnav with small screen and cassette,satnav tv with big screen and cassette and last of all large screen satnav,tv and cd.Look on ebay uk and for a better choice ebay.de.The basic cassette version is the cheapest,have you the alu frame behind the aftermarket radio you should have but if not you will need to install one no easy job as it requires removing a lot of the dashboard.

Plenty of advice on the bmw forums.

If it has flat pins then the cd player is seperate to the actual radio unit they come in two parts and are very expensive even second hand, if you still have the flap then you still have the frame it does not matter which model unit it is every unit with flat pins will work .Why do want to change back if you have a Sony,JVC,Apline head unit then most have a rear jack plug so as to be able to use the cars steering wheel controls for which you need a connects2 unit you also have the facility to play mp3 discs dependent on radio.

I have a e39 with a Sony radio with usb and mp3 plus a 6 disc stacker unit far better than the original.

I totally agree they make MP3 adaptors which fit into the stacker unit socket and display the tracks on the radio.

Have a look at these units on Ebay.de I am sure they will have them in the UK

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