I have been looking to design an ornamental vehicle steering wheel rim that illuminates from a light source, but I don't know if this have been patented or produced by the auto manufacturers yet.
How can I determine whether my design will infringe on existent design patents? What is the bar for design patent novelty?
Here's my take on it:
Step 1: Draw your design.
Step 2: Get a perspective on what being ornamental in nature is. Understand what being in the public domain means. I suggest the following reads, which will take you 30-60 minutes to read and digest:
You'll want to understand the basics of Egyptian Goddess v. Swisa.
From 35 USC 171: "Whoever invents any new, original and ornamental design for an article of manufacture may obtain a patent therefor, subject to the conditions and requirements of this title."
What does this mean? Well, there are two tests to perform, thanks to Egyptian Goddess v. Swisa:
Ordinary observer test "two designs are substantially the same, if the resemblance is such as to deceive an observer, inducing him to purchase one supposing it to be the other" --> Your widget looks like my widget.
"appropriate the novelty in the patented device which distinguishes it from the prior art." --> …however, both of our widgets look extremely close to generic widgets, so the scope of the design claim is significantly narrowed
So I would say, if you put your steering wheel on the shelf at Walmart and it was mistakenly purchased by a Walmart shopper who thought he was getting Company B's steering wheel, you fail step 1. Step 2 is for patentability, and that would mean if you made a wheel with christmas lights, well, both steering wheels and lights exist, so its not new and different enough in the field.
Sometimes, things don't need to look close in your opinion. Step 1 is infringed here because the Walmart shopper thought "I'm buying Crocs!", and step 2 was also infringed because Crocs were the first of their kind, arguably: 
From designpatentattorney.com
Other times, you'll scratch your head: 
From designpatentattorney.com
Step 3: Begin your design IP search. Another post in this thread identifies design patents for illuminated steering wheels. Look at the IP, and look at the class/subclass. I've pasted both below:
400 PARTS, ACCESSORIES AND ATTACHMENTS NOT ELSEWHERE SPECIFIED:
174 . Control element
175 .. Steering wheel
177 ... Element or attachment
In google patents, you can now filter patents by searching "D12/177", and selecting "design patents" only. Searching this way, you've only got three pages of results to begin filtering through.
You will want to try out a few different keywords around your design, look at the class, and repeat your search. Furthermore, you can use the term "inassignee:"ASSIGNEE_NAME" if you find a company that is heavy in design IP for your field.
Step 4: Review your results. Step 2 will teach you how to calibrate ornamental design while discounting the utility aspect. I suggest making a google spreadsheet and use the function "=image("url") to put in all the images that are similar, so that you can compare in real-time. Then you need to think of all the features (lights, location of lights, shape, etc...) and compare your design to references designs.
Also, of random note: remember that dotted lines are optional, meaning you can ignore them from comparing your design to the IP. This is important, as can be seen below. The reason Samsung infringed on Apple's design IP is because the dotted line on the side view shows that the back of the cell phone can be any shape, same with the dotted line on the push-button circle:
If you are starting from the beginning, this will take you 2-8 days of solid work to do properly.
Design patent is evaluated for novelty and non-obviousness, just like utility patents. However, what is being compared is the "appearance" rather than the functionality, in design patents.
It's hard to conclusively determine whether there is a patent out there on which your design may infringe. But there are some products out there for an illuminated steering wheel. For example: https://www.google.com/patents/USD623571 https://www.google.com/patents/USD526255
Seeing those should give you a feel for how similar/dissimilar designs need to be to be patentable.
In addition to designs, there are some utility patents on how to create illuminated steering wheels, like this one: https://www.google.com/patents/US6296380
As a side note, in-cab lighting is restricted in most states, and you should do your research on that aspect of law as well.
