89 Ford LTL 9000 Wiring Problem

I have 89 ltl 9000 with a 350 big cam cummins. Having problem with the brake lights. I checked the fuse and its fine and im getting power all the way thru. I have replaced the sending unit and thy still not working and without truck brake lites i dont have any on my lowboy trailer. And they were working fine and just all of a sudden quit.....Any one have any ideas....cuz im lost and i have spent all day chasing wires and crap......thank you

 

Could it be a ground issue?....im about at my wits end

 

i replaced the one on the airline manifold that is on the firewall in the engine compartment.....but i only found 1 ....i talked to another person and he said there should be a second for the trailer but i dont see it....after i replaced the one i see the trailer brake lites stay on all the time now....????????

 

yes....when i turn key on they come on and stay on to the trailer...without hitting brake pedal...but nothing on truck.....i hate wiring

 

no its a 2 post