Sircharles is right,detach faces to make the car panels which you can then create the gaps naturally and detail seperately.
Some details can be done with displacement (for example writing on tyre walls) it is simply your choice as to which to model and which to displace.
From the questions you have posted it seems you are not aware that you can reinforce a smoothed edge by placing another edge close to an existing one to keep the form.
The thing is that i didnt manage to add an edge without getting really messed up results, hard edges and unsmooth edges after scaling it to get the edges straight ... in 2014 this is way easier for me though because of the "insert with edge loop" option with the cut tool. Not sure if it would mess it up anyways.
I know that in the new modelling toolkit it's really easy to just re-create my mesh from the existing one, and separate the parts from where the gaps are and then combine them.
THANKS FOR THE ANSWERS 