Hello,
My name is Mario and I was born in 1990.
I do not remember whether it is an anime of the late '90 or the early 00'.
In this Cartoon the characters entered a robot from the back side.
The robot was kind of iluminated. They didn't have joysticks or stuff like that. Their own movements were the ones controlingv this robots.
If you are thinking of Gundam, please give me an exact Tittle because there are a lot of them.

Thank you for your time,
Best regards,
Mario.

The inside of the robot was kind of iluminated.

Sorry I misstyped it.

That's kind of a general description
But maybe

Pilot Candidate
a.k.a. Cadidate for Goddess