Pull the carpet and reveal the ECU. The ECU will have a location it is grounded to. Use the same one.

 

I spliced mine into the hazard lights 12v under the center console which is constant, grounded it on some random bolt down there next to the E-brake and put my gps unit in the same place under the center glove box. getting it in there though was a bitch. you gotta take both panels behind the seats out to peel up the glove box!