It's simple :
Run a wire from your Headunit's BLUE WIRE (REMOTE OR POWER ANTENNA ACTIVATOR) all the way to your trunk into your AMP Remote Terminal.
Then Run a Main Power Wire (follow Amplifier manufactor gadge specifications) from the Battery positive Terminal all the way back to your AMP's Power + Terminal, also make sure to use a IN-LINE FUSE of Proper Rating no more than 12inches away from the battery. Check your AMP Manufactor for Fuse Ratings.
Next pull back your Trunk Lining on the sides of the trunk and fine a suitable hole in the metal for your Ground Wire for the AMP. Use some sandpaper and sand the paint off the surface to bare metal. Attach your Ground Wire firmly to this point using a Bolt/Nut Combo, Then attach the other end to your AMP's Ground Terminal.
Then Run your RCA Cables from the Headunits SUB Pre-Out on the back of the headunit -or- REAR Out depending on the headunit all the way back to the AMP.
Also make sure to run your RCA's down the Right side of the car and the Power and Remote Lines down the Left side of the car.
Doing this helps ellimate Noise and interfearance.
If you have anymore questions let me know.
If your headunit isnt working because you shorted somthing its prolly fine, check the FUSE on the back of the headunit where the wire harness plugs in, If its good then check the Radio fuse in the Fuse box at the Driver Kick Panel.
Good luck and Happy Bumping!