I think you should get an idea from the dealer of what they think the car is worth now and what it will probably be worth when you trade it but what is going to happen if they give you a number but when the car comes in they tell your vehicle is worth less than expected. Are you going to walk away from a car you had custom built and waited nearly 6 months for - I doubt it. Unless of course, you order something which the dealer doesn't think he can easily sell because of your build. I don't know where you are but December is a very slow month for dealers so it isn't likely they will give you top dollar for you car at that time.