I'd like to add that 70 bucks is a pretty good price for a double din nav install if you were quoted it by a legitimate professional shop, especially for a unit they aren't selling you and one that is also used. If I had a dollar for every time a customer who has told me "I have everything I need, I just need you to install it" and then was missing parts, I could retire. Not saying you are one of these people, but just keep in mind that the other side of the coin from the shop's perspective is that it is typically a bigger hassle to install used customer provided equipment than it is new.

Also, if you are looking for recommendations as to shops to check out, post up your general location and we might be able to direct you better.