The Carnet is necessary on trips like a Transafrica or the Silkroad. For the Panamericana it is not required since a long time.
The ATA Carnet, often referred to as the "Passport for goods", is an international customs document that permits the tax-free and duty-free temporary export and import of nonperishable goods for up to one year. It consists of unified customs declaration forms which are prepared ready to use at every border crossing point. It is a globally accepted guarantee for customs duties and taxes which can replace security deposit required by each customs authority. It can be used in multiple countries in multiple trips up to its one-year validity. The acronym ATA is a combination of French and English terms "Admission Temporaire/Temporary Admission." The ATA carnet is now the document most widely used by the business community for international operations involving temporary admission of goods.
You have to deposite Money to cover the Tax Value of the Vehicle. Wikipedia has tons of informations about: https://en.wikipedia.org/wiki/ATA_Carnet
Also you should look at wikioverland for recent informations.
Carseurope.net for Europe and UK
Swiss ACS (Price for Members 220$, otherwise 310$)
Swiss TCS (Price for Members 220$, otherwise 330$)
German ADAC (Price for Members 195 EUR, otherwise 295 EUR)
As you can see on the Map of Africa, you can do a lot without the carnet. Even where it is required you had a chance to get into the country with a deposit of cash, what you can get back on the way out. With the carnet you dont had to carry that amount of money with you.