Hi,
Firstly, yes it is possible to do what your trying, but its not necessarily that easy.
I spent along time trying to get mine right.
THe trim panel, due to the size of screen you will want (for it to be visible and useful) will need to some chopping about. This isnt really a problem, but you will need to get creative with your method of mounting the screen in the hole you end up with. Due to the molding methods used, the back of the trim has a lot of awkward shapes to chop about and enable it to refit correcly.
Main issue i had in the design, is getting round the glare that is placed on the screen during the day. You will notice most vehicles have a shroud of some time around the screen. So will be worth thinking about that inorder to have a usable screen at all time.
There are plenty of options to run power etc fromt he headunit wiring so that is simple.
Running the cables from front to back isnt too difficult.
You will need to decide on whether you want sound from the system, as this will require a additional stereo loom part, just a plug and play, but will remove any sound output from your cd changer as it gives you an effective "aux in".
Apart from any of the issues, its a fun project.