I have exported a 2000 Boxster S from Canada to the USA.
It's quite simple.
You need to have registration-insurance papers of the car and some forms from the US border customs office before you can cross the border from Ontario to the USA.
Is your car originally from the USA?
Here is the website I used last year with all the info:
http://www.foreignborn.com/visas_imm/entering_us/7importingyourcar.htm
''Documentation
For Customs clearance you will need the shipper's or carrier's original bill of lading, the bill of sale, foreign registration, and any other documents covering the vehicle. You will also be required to complete EPA form 3520-1 and DOT form HS-7, declaring the emissions and safety provisions under which the vehicle is being imported. Vehicles that meet all U.S. emission requirements will bear manufacturer’s label on the engine compartment in English, attesting to that fact. For vehicles that lack such a label, the Customs inspector at the port of entry may require proof of eligibility to import under the EPA exemptions or exclusions specified on form 3520-1.
Vehicles that do not meet all U.S. emission requirements, unless eligible for exemption or exclusion must be imported through an independent commercial importer (ICI). EPA will not allow the vehicles’ release to the vehicle owner until ICI work is complete. The ICI will perform any EPA-required modifications and be responsible for assuring that all EPA requirements have been met. Some vehicles cannot be successfully imported or modified by an ICI, however, and in general, ICI fees are very high.
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Dutiable Entry
Foreign-made vehicles imported into the U.S., whether new or used, either for personal use or for sale, are generally dutiable at the following rates:
Autos. . . . . . . . . . . . . . . 2.5%
Trucks . . . . . . . . . . . . . . 25%
Motorcycles. . . . . . . . . . either fee or 2.4%
Duty rates are based on price paid or payable. Most Canadian-made vehicles are duty-free.
As a returning U.S. resident, you may apply your $400 Customs exemption and those of accompanying family members toward the value of the vehicle if it:
Accompanies you on your return;
Is imported for personal use; and
Was acquired during the journey from which you are returning.
For Customs purposes, a returning U.S. resident is one who is returning from travel, work, or study abroad. After the exemption has been applied, a flat duty rate of 10% is applied toward the next $ 1,000 of the vehicle's value. The remaining amount is dutiable at the regular duty rate.
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Free Entry
NONRESIDENTS may import a vehicle duty-free for personal use up to one (1) year if the vehicle is imported in conjunction with the owner's arrival. Vehicles imported under this provision that do not conform to U.S. safety and emission standards must be exported within one year and may not be sold in the U.S. There is no exemption or extension of the export requirements.
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Good luck
I would suggest calling the USA Border patrol customs office of your nearest border crossing into the USA before filling out all forms so you don't waste your time. Just double-check everything with them.