Why the men in Brown only stop pickups
I am sure you looked at the other story about Thai Police crack down and how they fail to do to do there job all the time. Well the problem with the traffic police is about the same. Most traffic police are from the rural area and they know that people that drive pickups or motorbikes are most of the time not from Bangkok. So if you not from Bangkok you do not know anyone in high places that can create problem for you later. Therefore if you buy a Benz or a BMW (the bigger the better) your risk of being stopped is minimal. (Yes that is why all Benz owners drive like idiots) Because you might be an important person and they do not want to risk stopping you. There is also a class difference in Thailand even if you not as a foreigners see it as much as in India but there is one and most traffic police come from a lower class so to speak.
When I bought my last car I had the option of a pickup and a SUV and the sales person even told me not get the pickup just because this problem. If I was to travel outside Bangkok at night I would be stopped all the time. The police does not like to stop foreigners either well some do but if you talk for example Italian with him for 5 min he will let you go.