No, you aren’t having a stroke. That ad does say “Ford Grand Marquis”. In Mexico, for some reason, the Mercury Grand Marquis was marketed as a Ford. But it retained the Mercury front badge.
Notice how I said front badge, because on the rear it came with a Ford badge, making it look like some weird Grand Marquis-Crown Vic crossbreeding accident.
Also interestingly, even though Mercury cars stopped being sold in Canada after the 2004 model year, the Grand Marquis soldiered on, being marketed by the Ford division instead of Mercury, technically also making it a Ford, and not a Mercury. Although they retained all the Mercury badging and commonly were referred to as Mercurys. The main reason for this was because the civilian Crown Victoria was discontinued in Canada for the 2000 model year and became fleet only (which also happened eight years later in the US), leaving the Grand Marquis to be it’s replacement.