be polite be respectful and don't come off as a know it all.
Dress nice but not necessarily suit and tie. It's better to be overdressed than underdressed.
DO you have a portfolio of your work? May not hurt to take it along as well as any designs or anything you can bring to the table for that shop
vehicle specific wiring is very tough to memorize for all the different cars out there. basic head unit wires are always the same color. that is easy to memorize.
things not to ask.
salary.
discounts.
breaks vacation etc.
Things to ask
average customer installation what does the average guy off the street come in for? how many installs on an average day.
average work day. what goes on during the daily grind.
alot of interviewing is developing a vibe with your interviewer. From there the conversation can ebb and flow as you feel it.