I looked into the Nexus7 but the lack of external storage was a killer for me. No matter how much I want to use the cloud, I will always want a good stockpile of songs on my actual device and I cant really achieve that with the Nexus. I also didnt like that the homescreen is locked to portrait mode instead of landscape, unless you root it (unless Im wrong on that). I have already procured another dash/radio trim piece to modify so that my original stays pristine so I am prepared to move forward on that part. I have some good ideas to make it all work and will start a worklog as soon as the damn need to have every comment approved by a moderator goes away.
I will take a look at that amp that you linked. I briefly looked at it and it looks pretty cool. My other idea was to look into single din aftermarket head units and see if I can hide one in the dash but I think they are all still the same depth as my current head unit. I was hoping that maybe they had grown smaller depthwise over the years since the last time that I had to shop for one, but I might be out of luck there. Still need to investigate that. Regarding the steering wheel controls, I was referencing my oem built in controls. In my research for this project, I found that if I remove the factory head unit, I will lose my warning chimes and ability to control the volume/preset with my built in steering controls, and would need an adapter that is about $150 in order to get them back. I found this when looking at replacement head units on crutchfield I think. Ill have to look into that some more. Im not thrilled about losing that even though the tab has its own volume control.
Unfortunately the iphone on tmobile is not natively 3g as it uses a different frequency than the one that tmobile uses for 3g. Recently tmobile added the frequency to their network that the iphone needs for 3g specifically for those of us that use an unlocked iphone but dont want at&t's ****ty prices, but its spotty at best and it will take quite some time to roll out to towers everywhere. Currently I get 3g in downtown cleveland pretty well, and in a few other select cities but in small areas. Its basically not useful for me at all yet. I am looking into a different phone as an upgrade so that I can have usable internet speeds while out and about, and I can always go back to my iphone if I want to in the future. I agree with you on the usefulness of the tablet being limited without that connection to the web though and thats an integral part of my build. In the meantime, I can use my mifi from work as long as I dont run up the data usage too high lol.
Thanks for the replies! Your knowledge is definitely a massive help in planning and completing a project like this.