best way to add spdt on gmrc 01?

I have found tons of great information regarding relays on the12volt! I love this website!      First time using the metra gmrc-01 interface and of course i need to add a relay. My head unit requires more than 3amps.... I have the interface and wire harness wired up allong with the relay and im ready to install. Question- im running the +12volt Batt wire directly from the batt (fused of course). CAN I TAP INTO THE GROUND WIRE FROM THE EXISTING VEHICLE RADIO HARNESS or should i find a place inside the dash to do a proper body ground? 2007 grand prix. Thank you thank you for the help

     

Which wire are you worried about the 3 amps?

Thanks for the responce, Im good on the 12+ wire, the stock 3amps that the interface provides is not suffecient to run my radio hence the reason im adding the relay, The power supply is comming directly from the batt (i did use an inline fuse)- all good with the power supply. Thank you kindly Question is regarding the ground wire for the relay. Can i ground the relay by tieing directly into the ground wire on the vehicle's factory wire harness? Can i be lazy and safely ground the relay to the ground wire right there in the stock wire harness or is it better to just find a place to make a nice body ground somewhere behind the radio/in the dash. I suppose i could just test the existing ground- see if it is sufficient and if it isnt..... find one that is...? Anyone who has experiance adding a relay to the metra gmrc-01 interface- please chime in on how you grounded the relay. Thanks allot guys

Are you worried about constant or switched power?