MrDerek's post in Spotify? was marked as the answer
Hi Guys, Found a solution to my problem so thought I'd post it for anyone who want's to know. The answer didn't seem to be on the internet anywhere, nowhere obvious anyway. Alpine units - only tested this at halfords but if you simply plug in your iphone via USB, spotify plays through the speakers. No control though, all through phone. Pioneer app radio2 (sph-da100) - This is what I went for. plug the phone in via HDMI and again your phone audio plays through the speakers. The better solution however, is to get a rooted android phone, download an app called ARLiberator and you can basically mirror android onto the headunit with full control. So any apps you have (including spotify) can be controlled directly from the unit. Hope this helps someone.
