I like hypothetical questions like this, because they make us thing about how to handle situations before they arise, so that we know how best to handle them when they do.
HOWEVER...they also allow us to better handle all the OTHER aspects as well.
For example, the proverbial "gun on the dash". That's as bad as, if not worse than, the proverbial "gun on the seat next to me".
The dash is the absolute worst place to put ANYTHING unsecured. Toss your phone up there and drive around a bit and see what I mean.
If you're going to tote a firearm around in a vehicle and it's not secured in a holster on your person, then take just a teeny, tiny amount of time out of your very busy schedule to figure out how to tote it around SECURELY.
First turn you take with a gun on your dash and it's gonna slide all the way to the opposite side and end up off the dash somewhere.
First time you have to hit the brakes hard with a gun just sitting on a seat and the car will be on the floorboards somewhere.
This isn't rocket science, and it's certainly not meant to belittle people by posting this. It's a straight up, valid concern that ought to be reasonably addressed. Use the glove box, console compartment, or perhaps some form of securely mounted holster in a convenient loction.
NOW...that said...if you're legally open carrying in South Carolina, which is no longer a "must inform" state by the way, then if you feel the need to inform an officer simply do so EXACTLY as you would have BEFORE open carry was legal and SC was still a "must inform" state.
To wit, calmly make a statement (which does NOT start off with something like "I have a gun") which cites state law on the matter.
If you happen to have a concealed carry permit (not required for open carry, but let's pretend you're like myself and have a CWP):
Hand the officer your CWP with your DL/registration/insurance and say something on the order of: "Officer, I have a concealed carry permit and I have a firearm in the vehicle."
If you don't have a CWP, something like "Officer, in accordance with state law on open carry, I have a firearm in the vehicle."
Of course, you aren't required by SC to inform at all, which means you have the leeway to decide whether or not you want to based on your specific beliefs and circumstances. If the firearm isn't visible, then maybe you will choose not to say a thing. If it is visible, you may choose either way, or to wait until the officer addresses the situation.
IF THE OFFICER ASKS IF YOU HAVE A FIREARM IN THE VEHICLE, this can open up a can of worms depending on how you choose to respond (and, of course, the officer).
If you choose to answer, THEN DO NOT LIE. If there are no firearms in the vehicle, the answer is "no, sir/ma-am". If there ARE firearms in the vehicle, the answer is something on the order of "Per state and federal law, I have a firearm located in the (name the place)." If the officer ASKS you a question and you choose to lie, the fact that you lied to the officer about this could make things worse on other matters...especially if it's something that ends up with you in court. An objectively provable lie calls your character into question on everything.
If you choose NOT to answer, then this should be clearly stated as exercising your right to remain silent and then keep your trap shut...you cannot exercise a right to remain silent only on specific issues while flapping your lips all day long on other matters. "I'm sorry, officer, I don't answer questions and I choose to exercise my right to remain silent."
EVERYBODY WILL HAVE AN OPINION ON YOUR QUESTION. Ultimately, "you do you". If you are a person who honestly believes it's in the best interest to inform LEO on a simple traffic stop that you have a firearm in the vehicle (as in the open carry scenario), then have at it. If you are a person who thinks it's none of the officer's business and you don't see any reason to inform the LEO if the question never comes up, more power to you.
Be calm, be plainly spoken, and make sure that you start off the conversation in a manner that clearly informs the LEO that what you're about to tell him is "per the laws" and then that you have a firearm in the vehicle.