Talk to your union if you have one, or a lawyer.
You should have grounds for wrongful dismissal.
You might be held partially responsible for the damage, because you do have free will and could/should have refused to drive. Your liability will depend on whether you have any history of fainting, especially under similar circumstances.
If there's not history, and you feared dismissal if you refused the trip, then they don't have much of a case.
First, sit down and document EVERYTHING. Note the date and time you had the conversation with your boss, where you objected and stated your symptoms. If there were any witnesses, note their names and get their phone numbers and addresses for your lawyer. Ask those people if they will attest to what you said and your boss' insistence that you take the trip anyway.
If you're not unionized, you may still lose your job. You might want to play it smart and ask for letters of reference from anyone possible now, before the sh.. hits the fan and you're out. Don't accept a promise of verbal reference, you never know what they might say. If you get it in writing, they'd have a hard time justifying a negative verbal reference after giving you a positive written one.
Definitely start looking for other work now regardless of how this turns out. Your employer is clearly unethical and unworthy.