Interesting case. Clearly the stop was based on a pretext, the notorious broken tail light excuse. Cops also do not routinely ask to search your vehicle when they stop you for an equipment issue. While db is correct that the Court said the motorist consented to a search, it probably was not a factor in the decision(I didn't read the opinion). The police have considerable discretion to conduct warrantless searches of vehicles after a lawful stop.
Still, if you're ever in that situation, never give consent, at least if you have contraband. Failure to consent to a search cannot be used as evidence against you or as ground for an arrest.
The case seems odd to me because it appears to decide an issue that was unnecessary to the decision, which courts usually try to avoid doing. Even if one broken tailight is not a violation of the law, a cop can still stop you and tell you it is broken. If he then decides he wants to search your car, he can ask for consent. He could have done the same thing if you were stopped at a 7-11. So we are back to DB's point regarding consent. I don't see a constitutional issue there.
The question of whether the police can rely on an erroneous albeit reasonable interpretation of the law has typically involved issues of criminal procedure, eg whether they can conduct a warrantless search of a house. This turned on a simple issue of state vehicular law, not some exotic question of constitutional interpretation. It does not seem unreasonable to me to require the police to know the state vehicular equipment rules. I don't understand how the Court could say it was a reasonable mistake when there is a clear cut law.
At the end of the day, I don't see this as a very important case at all. Prior cases have established that motorists and pedestrians have very diminished rights versus people in their homes. The stop and frisk issue in NYC is an example. Is having your car searched any more intrusive than being frisked on the street by a cop when you are minding your own business?