Do I need to cut a hole in my rear panels to fit the rear sway bar in a EG hatch? would a dremel tool work?

 

depends on how you intend to mount the bar.
the rear subframe... wait.... panels?
do you mean strut bar?

sway bar attaches to the lower control arms and to the subframe to keep the more level during cornering.
a strut bar attaches to the upper and lower suspension piviots to brace the chassis.

a strut bar in an EG hatch requires cutting unless you buy a set that doesn't require cutting. ie the ones that lift up the mounting points to above the panels.
the plastic is really soft, a utility knife makes short work of it.

 

ithink you are refering to a rear strut bar...
if you are...

there are lots of options for rear strut bars...those that require cutting and those that rise out of the factory hole.

if you cut you can do it with a utility knife or fine hand saw, etc...dremel tools are great for cleaning up the plastic.