Where is the ECU located on a 2005 cobalt ss/sc. i need to know so i can hook a shift light up. according to summit there is a white wire i need to tap into for the rpm signal.

 

Look for the big heat sink directly infront of your fuse box.

However I have to say this, If your here asking where the ECU is, I wouldn't be tapping anything into any part of it if I were you.
Know your limits, there is such a huge risk of doing serious damage.
I can do alot electronic wise to a car but the ECU is one place I would go, have it done by a pro.