I kept my factory head unit too, lot of features already in it so why not keep it..
this is a good thing to use,, it includes a harness that interfaces directly between your radio and speakers so you can add a 4 channel amp without cutting any factory wiring,you just have to run power from your ignition..
now you can use the included adapter to convert the signal to RCA amp signals or you can do what I did and modify the harness to send speaker level inputs to an amp that will accept them,,, the later is good because you don't have to mount the converter and run separate power and ground lines to it,, plus find a place to mount it, there is not much room behind the stock head unit either,,
Also I atrongly suggest that you get a 5 channel amp, especially with your convertible so you can add a Sub later if you desire, I regret only using a 4 channel because now I want a sub.. although it's not as big an issue in my coupe because it has 6x9's in the back which give plenty of bass.
also you can use a higher quality signal converter like this if you like, but I decided not too because the amp had lots of setting on it's own anyway and using this type of converter only makes the install more complicated.
AudioControl LC6i (Gray) 6-channel line output converter for adding outboard amps to your factory audio system at Crutchfield.com
and of course the most important thing is to replace the speakers, that by itself is the single best thing you can do for sound quality, they put the cheapest pieces of junk they can make in at the factory.