Ants are attracted to any types of food -crumbs, sugar, blood, all kinds of stuff. You're going to have to roll your sleeves up and start cleaning. Something like a can of coke spilled in the back foot-well will probably your cause. They won't move in for the fun of it -they have been attracted.
Check everywhere fluid can pool, check the spare wheel etc. and clean absolutely everything. Aftershaves, air freshener etc. can also attract them.
If you suspect spilt fluid on the carpets, it might be an idea to rent a carpet cleaner from HSS for £50< as this will help remove the detritus, not just push it around.