I built an aluminum sheetmetal plate that I attached to the front of my seat support. Then cut a hole and mounted the stereo under the driver's bucket seat. Bought a stereo with remote, so I don't have to be reaching down to do channel or volume changes.
I know this is a really old thread, but I thought I would bump it to see what if any other ideas were floating around.
2-wire input to the volume control on the stock radio, 1' stereo cable to Ipod, Ipod in ashtray, simple.
just get a unit from Secret Audio. Its just a black box with all the electronics in it that can be hidden anywhere. Then all the controls are on a little wireless remote you can velcro/mount with two sided tape/magnets or even wear it like a wrist watch...etc. wherever you want it.It has inputs/outputs for all the modern players, amps and even has a CD changer available. Then you can leave the stock radio in the dash for appearance but have all the modern audio out of sight.
RetroSound Model One unit in-dash but with the original 1959 Cadillac knobs installed on RetroSound's rotary encoders. Very happy with that. Although liking the looks of the new Model Two - has user customisable back-light colour - so would be nice to set it to a softer yellow glow for a more aged feel - and they also did away with the print on the buttons that also looks modern.