It was kind of a bone head thing to do. I went to buy a new alpine today but know-one in town had any harness

i do have one question every search i do one here says use the metra 70-5520-1 harness but it says that harness is for shaker system? and premium system. my truck has the mp3 and 6 disc player. also the scosche fdk11b is another harness i found on here.
Is that the same harness different brand?
thanks again for all your help
also let me clear this up in my head.
when i buy a new HU,
1. hook up the new harness leaving all speaker wires free?
2. hook up new speaker wire from amp to the truck side of the harness?
3. run RCA's to amp.
4. install new HU.
Also what does that smaller harness do that was also hooked up to the stock HU? is that for the dimmer and stuff?
one more question, does the Preamp output voltage matter? pioneer puts out 4V Alpine 2V...