Display everything . It really depends on how much space you have . It needs to be clean, organized, customer friendly, priced, and everybody needs to be involved. I would have video cameras recording the area nonstop. I don't expect much theft from customers but you will have it and you'll need to know who it is. The key is the customers have to be able to pick it up feel it.
Displays are not the forte of most parts managers or parts department employees. The best displays I've ever had were done by other staff in the dealership. I suggest looking at the men and women on the sales floor and the office for artistic talent to help you with this. I have also found outside vendors can be very helpful. Some will even supply and install shelving and wall racks if you let them display and sell their products.
I'm also impressed by the fact that you change your displays quarterly.

What I said display everything I even pull out the obsolete runningboards. They look great on display, and I am able to bill them to advertising so they're off inventory as well .