1995 GMC Truck Brake lights do not work on tool box body

My truck is a 6.5L diesal. I have a reading utility body instead of a regular bed. All lights and turn signals work including running lights. Just the brake lights do not work. Also my third break light doesn't work either. I have checked power at the third break light and I have power at the 4 pin connector on the third break light. My break lights on the reading utility body are sealed units made by Grote. I'm stumped? I can't even get to the break light switch. It looks like its buried in a steel casket, God its deep under the dash. All fuses are good and I have no tow harness connected right now. Any help anyone can provide will be greatly appreciated. Thanks in advance and happy thanksgiving all.

Mike