2002 Venture brake light problems

The lower brake lights on my Venture aren't working. I've already swapped out the circuit boards...bulb filaments on all of the bulbs are fine...no signs of schmutz on the connections, and all of the other signals work just fine. The high light at the top of the tailgate works fine. Any suggestions on what I've missed?

Thanks!

Mark

check thru floor harness for corrosion.....