There's two main ways I remember of getting there. One is by going into the movie theater, and in the movie room, you should see a propaganda poster for C17 on the wall. If you're a CP, you can just hit e on it and it'll open. If you're a citizen, then you have to have it opened from the inside. After you go through it, climb down the latter and just go straight forward. You'll find a garage door that leads straight into the rebel base.

The other way is accessible by anyone. Go into the Grizzly Grotto in the slums. If you go really far to the right, you should find a bathroom. Go in the bathroom, into the right stall, and click the wall. The wall will open. Go down the ladder and through the basement you're in. Once you exit, you should see a door. Go straight to the right, don't go into the door. Keep going along the path as straight as it lets you. Before long, you should see a garage door. Click on it, and you'll be in the base. Further more, at the left of the base should be two vending machines. Click the one on the right, and it'll open up, leading to an even more secret base. That's the main part of it. If you want to go to the jail cells, find the room with the furniture and TV, and click on the picture that's on the wall. It shows a picture of a hand holding the lambda, and says "Reborn". Click on it, and you'll be there.

Hoped this helped :3

Thanks Rodger!