Here are the manufacturer's recommendations:

Here is a link that might be useful: Sanford Customer FAQ

You would have to test or be careful it was okay to use on carpet, but Clorox 2 , mind you I said Clorox 2 gets all kinds of dyes out of clothing, who knows, maybe on carpet?
Or...I have this carpet/upolstery (sp?) cleaner from Oreck vacuum store. It has also worked pretty well for deep cleaning.

As an artist I'd say find out what kind of solvent and dye you're dealing w/by looking up the type of marker. E.g. alcohol may be the solvent and may take up the color. if you can tell a professional cleaning co what the type of ink is, they can probably get it out if you can't. I used to take out stains on clothes with airbrush cleaner, paintbrush cleaner, alcohol, whatever worked. Bleach will also remove the carpet's color, you need to use something that won't change the carpet, just dissolve the ink.

Are you talking highlighter? If so try this site.

Here is a link that might be useful: highlighter stain removal