The best training someone can get for being a service adviser is from the inside.
How would they like to be treated when they are receiving service. Look at some of the ways people have treated you. Did you like it or did it make you angry. Remember the good things, but don't forget the bad.
Being empathic is one of the best traits a service adviser can have. Feel what the customer is feeling. If they have a complaint that sounds crazy to you, don't blow it off. Listen to them and try to figure out what they are trying to tell you. Remember, they are not the professional, you are.
I'll give you an example. Many years back, I worked in a GMC, Pontiac, Buick store, I had an older gentleman who complained that whenever he got off the freeway and pressed on his brake, his car accelerated instead of braking. I had technicians take his car out and found no problems. I test drove it myself and found not problems. It wasn't until I drove with him that I noticed that he turned on the cruise control when he used his turn signals that I figured out what the problem was. Turning the cruise off fixed this problem. This is what I'm talking about.