Always use lots of photo references in conjunction with your blueprints when modeling an existing car. Photos can reveal much more about the car's surface than blue prints alone.
For trickier surfaces, it is good practice to create a "base-mesh" that is as smooth as possible before creating the final mesh. Then a final mesh can be modelled over it and shrink-wrapped to it to create a more perfect result.
A good way to create the base-mesh is to model a larger portion of the car ignoring all panel gaps and finer details. To ensure that the reflections are as smooth as possible, carefully plan the topology and keep the vertices relatively sparse. When it is finished, you will usually need to bump up the number of subdivisions to a high number, usually 3 or 4.
Pay special attention to how the reflections flow across the surface. It's important!
You would model more than just one panel for the base-mesh, typically the whole side, rear, or another large area. I only modelled a quarter panel to save time.
Once the base-mesh is satisfactory, you can model final panel meshes over the top of it using the Shrinkwrap modifier to keep the pinching in check. You can now include all of the panel gaps and cutouts without constantly worrying about nasty surface quality!
To make the panel gaps nicer, you could use the Solidify modifier set to "Only Rim" to create a lip around the edge followed by a Bevel modifier set to bevel by angle.
Obviously, this isn't the quarter panel that you're modelling, but the same principles apply.
There is no single answer. There are a million ways to model, so start modelling and watching tutorials! Develop your technique!
I can't magically place real experience and hours of modelling into someone's head!