In my experience with trading, you can usually get a small trade in bump if the parts you added are parts that Ford offers. As this will be towards their selling crowd. Things such as Window Tint, Side Scoops (Mustang Side Scoops - Pre-painted (10-14 All) - Free Shipping
), Window Louvers (MMD Classic Mustang Quarter Window Louvers - Pre-Painted 71316 (10-14 All) - Free Shipping
), and anything else that is in their purchase brochure of accessories.
When I traded my 2011 V6
, it had the louvers, engine cover, strut tower brace, and a few other small items. The dealer said this bumped my trade by almost $1,000. Car bluebooked at 17k but they ended up giving me like 17.5 for it. Really surprising. And that's the ONLY reason I traded. Had they given me the normal offer of like 12-14k, I would have declined.
When I traded my 08 Mustang, I had added the tri bar pony fender badges, the pony floor mats, and a few other things, and they also gave me an additional 500 for trade value for these because they said these types of items they can market.
Now if you get like say a Magnaflow Exhaust or some random stuff, then yea, they won't help at all.
When I spoke with the dealer, they said when they look at your car with you, they expect you to try and sell it to them. Point out the nice things you have done that is "Tasteful" and tell them about the maintenance history. Dont say you tuned it or added a BBK Throttle body to it. Because this is not stuff in their market brochure.
Here is their online brochure: 2014 Ford Mustang Custom Accessories | The Official Site for Ford Accessories
Lastly, it all depends on your dealer. But it doesn't hurt to mention the tasteful things (if any), that you did.