Craig is absolutely correct on the law as far as lights go!
Now your not the only one who has been mistaken/mislead as to if the pulling vehicle lights can be seen it's ok! (and a lot of times cops will over look trailers not having lights IF the pulling vehicle lights can be seen, just depends on how he is on his ticket quota!)
Shoot down here in KY/TN nobody uses trailer lights! But that don't make it legal!
Also as Craig mentions the mixers/small equipment deal he's right cops just turn their heads and ignor it or they'd be writing way too many tickets!
One other law I'll point out is that anything or any part of a vehicle that sticks out 4 feet past the rear end and light, Must either have a flag or light on it!