To get ANY kind of quality sound out of automotive speakers you need to run an amplifier. If you don't want a stereo head unit showing in the vehicle and you plan on playing music through your Iphone or an Ipod there is another choice.
You will need a few simple components and I'll give you those links here:
This device would be mounted under the carpet and could exit from the e-brake boot. A foot of cable would show which would allow you to hold your Iphone/Ipod in your hand to make song choices etc.
http://www.crutchfield.com/p_1...35-RCA.html?tp=47586
That cable would be wired to the battery and a ground so your Iphone/Ipod would charge while plugged in. Then the RCA cables from that device would plug into a separate volume control to allow you to control the volume of your Ipod/Iphone without picking it up. It would be connected to this setup which could be easily concealed under the dash within easy reach of your fingers while it rests on the gear shift. It also requires a power source and a ground:
http://www.crutchfield.com/p_1...-CL-RLC.html?tp=3091
The volume control from the link above is plugged in directly to the RCA inputs of the amplifier. With this setup you plug your Iphone/Ipod in to the adapter that is connected to a volume control that is connected to your amp. No modern day stereos are mounted in the vehicle, no holes in the dash, and nothing under the dash except for the small volume control knob.
Cost for these parts is relatively minor in comparison to the RetroSound unit. $100 for both sets of cables. If you have an Iphone 5s or newer you will need one more adapter that is $40.00.
Here is a Porsche Speedster with the same exact setup:
http://www.youtube.com/watch?v=umnczGIcdJk
Just for reference I mounted a JL Audio 500 watt under my dash against the front firewall and you can't see it unless you look under the dash. The other options is mounting the amplifier in the battery well at the front of the vehicle. The cables appear to be long enough for that as well.
The only caveat is that you won't have as much control over the SOUND of the music. With a stereo you can more definitively adjust the bass, treble, mid-range, etc with a stereo. Your Iphone does not allow for any control over the sound and an Ipod has preset sounds such as Jazz, Rock, Hip-hop, Extra Bass, Extra Treble, etc.