If I use a proprietary name in a work of art/literature I create, do have to pay them?

You can use proprietary names in a written story but you may not be able to use pictures of products that may have copyright protection or patent protection.

As long as the product is "in the background," there shouldn't be a problem. You're just providing free advertising for the company. However, you need to be careful that you don't blacken the image of the product or company (ie: only bad people drive Fusions).

If used properly - this would be a "fair use" of the trademarks - and you do not need permission.

No. What you are proposing amounts to free advertising. Most mark holders would welcome the free advertising as long as it is not disparaging of the marks in question. From a trademark infringement perspective, the mark holders tend to take issue with your use of their marks if you creating a likelihood of confusion in the marketplace by trying to pass of your goods using their marks. So, as long as you are not trying to sell a that looks like a Ford Fusion, using Ford's trademarks, or a soda that looks like Coke using Coca Cola's trademarks, or music that sounds like Metallica using Metallic'a trademarks, you should be fine. An acknowledgement that the proprietary marks you have used belong to their respective trademark owners is always a good idea. Of course, you should consult an intellectual property attorney to fully explore your proposed use of the marks in question and then properly advise you in view of relevant case law.

If in your book the hero drinks a coke there is no problem. However if he disparages Coke therein you may have problems.