I have an Alpine using a ISO connector to the HK upgrade system (well until a few ago when I changed the speakers). I'm now looking to upgrade the Alpine to a newer one to play from my iphone 5 and Android or 6 when I change phones.
This one seems great value at £69
http://www.halfords.com/technology/...-drive-less-head-unit-with-bluetooth-and-ipod
Built in Bluetooth and comes with mic for hands free calls.
What I really like is the Alpine app for iPhone and Android phones - let's you strength music from phone/internet radio stations.
Also has the USB on the front to charge phone and stream music.
It's mech less so no CD or tape, only plays via Usb or Bluetooth.
Has pre outs for sub and sub level controls (on my HK set up my current Alpine let me control the sub volume)
£69
As Gaz says you just need the ISO adapter and plug and play.