98 savana 3500 box van no brake lights

I saw some other brake light issues but none of them were the same as mine nor did they help me. So I'm posting new thread. 98 GMC Savana 3500 Box van. around 200k miles, The blinkers and brake lights do not work... BUT the running lights do. I would assume its a ground or short but I have no idea where to begin. The pigtail connector at the very back end of the truck that controls all of that is solid connection and no water or corrosion. My first guess is brake light switch under the dash. my second guess is short or bad ground. But I have no idea where to even start looking for a bad ground. The wire runs all the way up to the front of the van. Grounds and all.

Thanks In Advanced.
James

Basdcs, start by checking for 12v at the stop lite switch and 12v coming out of it.