Are you trying to create an ad that is basically black and white, but certain elements, such as a car, are magenta?
Is the ad of a photo, or an illustration or just text? If it's a photo, you'd need to a) convert the entire photo to B&W, then use the lasso tool and go through and re-add color (magenta) to what you want colored; or b) use the lasso tool and pick what parts you don't want magenta and convert those to B&W leaving the magenta element untouched.
Both would be time-consuming. If that were the case (I'm in editorial not advertising), I'd suggest a0 converting the photo to greyscale, then doing a color (magenta) overlay over the parts you want colorized; or b) converting the photo to greyscale, then doing a clipping path over what you to be red and then importing the original magenta elements (which would also be from a clipping path).
If it's in illustration, you can just use the paint brush and fill in the areas you want to be red.
However, if there are any other color elements (cyan or yellow) on the ad, then those will still reproduce when printed, but they wouldn't be used when printing, so you could have some problems.
What are you using to print?
And my honest opinion (from what little we know) is you're better off just running the photo of the car(s) in black and white, then using magenta as a color headline/text superimposed onto the photo.