Im fitting an amp into my RT at the moment (but as youll see on another thread its not quite going to plan!). Running the power cable is fairly simple on this car as there is a perfect spot in the bulk head that pops out at the corner of the dash that you can run the cable through.
Its worth having a go yourself, Ive never installed an amp before, and although Ive got a few issues, I think I know where the problem is. If its a three door then you can take the rear seats out pretty easily which makes the job easier.
Fitting the head unit and front speakers is fairly easy, its just a case of having the patience.
As fred says, the tweeters are in the dash, Ive got coaxials in mine, not components (which have the seperate tweeters) it looks pretty easy to run the cables through the door though.
If all else fails, get someone else to do it, but it will be v. expensive (places like Halfods charge £30 just to fit a head unit, which is only a ten minute job).
Rick