This is outrageous!

However, what the driver probably did was to turn ALL his lights on and off again -- including the lights on the back of his car. So he effectively signaled the cop car behind him.

I don't know about all cars, but on MOST cars, if you simply pull the light stalk all the way back, only the head lights will flash on -- and on the BRIGHT setting too.

So you don't end up telegraphing what you're doing to anyone behind you.

It is NOT illegal to warn fellow drivers of a speed trap or  a stop to search cars and trucks for drugs or papers. 

It SHOULD be illegal under the Constitution to have your truck or car searched without a proper search warrant...not a John/Jane Doe blanket search warrant for everything.  And, in the case of a warrant and search of your car or truck, you should be able to demand your lawyer be present before the search starts.    Miranda for the road.  AND, the citizen should be able to video record any of the search by the cops as evidence and turn it over to his lawyer. 

What's good for the goose is good for the gander.