Quote: Originally Posted by
elimenohpee 
I have 2 quick questions:
1. I want to tap in the acc wire behind the radio to power on the psu. Now, should I tap the wire coming from the car's original harness, or from the acc wire coming out of the hu? Or does it matter?
No No No!
Only use the Acc wire in the stock harness for the sensing in an intelligent PSU. The guage is not thick enough to safely provide enough current to a PSU. Run a decent wire from the battery or fuse box for safe, clean power.
Quote: Originally Posted by
elimenohpee 
2. Should I use a relay when connecting to the acc line?
Not required. see answer 1. You can use a relay in conjuunction with the acc feed to power on a PSU through your new wire if you wish. If you don't understand what this means, please read a little one basic wiring and componenets before proceeding.
Google is your freind.
Good luck