Yeah well... this instance would be a fairly cut and dry case of wrongful termination (especially if the job did not stipulate anywhere what kind of car you had to drive to work).
Of course, it all depends how strongly the person feels about it. I wouldn't suggest purposefully pissing off an employer enough to fire you or pressuring him this way as it would mean a bitter employer for being "pushed" into something. Wrongful termination cases are often involving claims about sex or race. These case generally are fake, at least I think so, but often are successful because of the whole "never blame the victim even if they're lying" mentality.
If you have this threat of being fired in writing that'd be a good start (and the reasons). Just be sure to record any conversation you have with your boss about this (cell phones have record buttons nowadays... makes for a very open/shut case).
Knowing how incredibly stupid bosses are about laws, I'm sure he'll be crazy enough to run up to you and say "I told you not to drive that F***ing car back here... you're fired!" or something... that would be fairly incriminating.
Problem is you have to prove two things:
1) that there was no written rule governing what car employees can drive there (even if they created one after firing you, you wouldn't be liable for that rule). Be sure to have an updated copy of the employee handbook/rulebook if one exists. If one does not exist then they have the issue not you... just be sure to keep any documentation of threats for termination.
You might be able to claim the car is your only form of transportation, which is often a right protected by state law unless the job requires extensive travel time. I just wouldn't bet the farm on that. You will really need to hope for a very strict legal minded judge (legal philosopher type) to really agree with you if that's your primary defense.
2) that he fired you for driving a registered/approved car to work and not for any other reason (why that tape recorder is very necessary on the day of termination)
You need both of these to be proven in order to win such a case usually.
My suggestion is:
Document that the car is your primary mode of transportation by necessity (ie. you could not drive any other car to work, for fiscal reasons or simply due to not owning another car). Be sure to send this to your employer, even if they just toss it in the garbage later.
Be sure the current handbook (if your company has one) does not govern what cars may be driven to work by employees
If you can find a handbook from when you were hired that would be an incredibly useful thing as well
Record any conversations with your boss
Keep all emails/threats etc if your boss is stupid enough to write them down
Often if you see a law suit coming, or even one being possible, you can make the case extremely threatening (ie. most defense attorneys looking at evidence such as that would push hard to settle).
