Which is stronger/lighter? Plastic(from a car interior) or fiberglass same dimensions? Thanks!

Depends on your intended use/material requirements.

There are also different types of plastic - different ways of forming, etc.

I think in my old civic, I am going to be doing some fiberglass parts such as armrest and console. ANd i was wondering if it will be as strong and atleast the same weight. thanks

....it breaks. Plastic may not be as statically strong, but instead of failure (cracking, disintigrating) it will flex or simply lose some of its form. Breaking or fraying into pieces that may cut you, as FG will, is unlikely.