First: I am not your attorney and the following advice is not to be considered legal advice.
Second: If you wish to obtain legal advice, go to a lawyer, preferably one who specializes in auto-accidents.
Third: $40 is not worth much trouble. If you have the time, you can write letters to the other insurance company demanding the entire amount (presumably $400) explaining your break lights are in perfect working order.
Phoenyxx:
I bet if you somehow looked into their practices and found a record of them shorting folks 10% all the time, you'd have a class action lawsuit to push... you'd be a hero.
That would be extraordinarily difficult to prove - to have a class action, you have to have a class of plaintiffs "certified" by the court. To get class certification, you generally have to have common facts. The opposition will argue that each case has its own, unique facts (in your case, the break lights were out, in the other case, the person swerved, etc.) Accordingly, you are unlikely to get class certification absent finding a memorandum from the insurance company that says, in effect, "Screw every customer for 10%!" And of course, to have a chance to find that memorandum, you have to first file the lawsuit (which in California would cost you upwards of $350 just for the filing) and then you have to propound discovery (depositions, interrogatories, requests for documents, etc.), you have to fight the inevitable discovery motions ("we are not going to give that up - its irrelevant/privileged/not-available") and then, maybe, you find the "smoking gun" memorandum ... but of course, by then, you are in the hole for a LOT of money and a LOT of time and a LOT of heartache.
And ultimately, even if you prevail, the only REAL winner will be your lawyers who will get a chunk of the ultimate judgment's value. As the lead plaintiff, you might get (depending on your state's laws) some token piece of the action - but trust me, there are so many "ifs" between you and that money you might as well go buy some lotto tickets with the $350 you were going to spend filing the complaint.
Now ... you do have another recourse for your own $40, and that is small claims court. In California, it costs $50 (I think) to file a small claims case, but I believe if you prevail you can get your costs from the opposition - so, you stand to gain all the money.
Here is the downside - lawyers cannot represent people in small claims court (at least in California). And this is only a $40 matter - so - in all liklihood, the insurance company may not even send anyone to the small claims hearing. This would likely result in you getting a default judgment (i.e. you "win").
Of course - so what? Now you have to collect. So you send a copy of your judgment to the insurance company for (best case scenario) $90 ($40 damages and $50 filing fee) or maybe $450 (if you have to sue them for the whole $400, plus filing fee). And then ... maybe ... they ignore you. You would then have to pursue collecting on the judgment, and that is no easy task in some cases.
So - whats the conclusion? Well, this is not legal advice, but in your position, I would demand the entire $400 in writing. If they refuse, I would probably just knuckle under and take the $360 because $40 just isn't a big enough deal to go to court over, even small claims court (unless I am just soooo freakin' mad I cannot contain myself).
Not too long ago I had a very similar situation - while I was parked, in a parking space, car turned off, a taxi-cab hit me. The taxi-driver wanted to settle out of court and off the record, but he offered me only about $600 and my body shop estimated the damage at $2300. So I said no and reported the incident to my insurance company who took up the matter with his insurance company. His insurance company came back with a "walk-away" offer (meaning we both just walk away) under the reasoning that we (taxi and I) had backed-up into each other - now, that excuse might have been reasonable (even if untrue) but ... the dent on my car was not on my back-end, but on my driver's side door! Unless my car drives side-ways (which it doesn't) I could not possibly have "backed into" the other guy.
My insurance company pointed this out, and, after 3 months, the other folks finally paid off on the whole thing. No court needed - just some letters and phone calls. And in your case, I think letters and phone calls are all its worth.
Just my opinion though.