This is the only thing I have found that is a realistic option:
http://www.amazon.com/dp/B00B52LLJ6/...883V0UDD&psc=1
If you use an Iphone, it will do just about everything a Combox retrofit will do. If you use an Android, it will work for audio and do a great job, but you won't see any graphics on the stereo display. It will allow you to use volume and skip songs with the steering wheel, but no visual. Not perfect, but a better solution than paying close to $1k for the retrofit.
Do a search on this product and you will find very positive reviews.