1994 gmc serria

ok i only have tail lights if i tap my brakes after i turn on my lights but if i use my brakes i only have the third light for brake lights and no tail lights turn signal flashes fast if either brakes or head lights are applyed at the same time everything except the tail lights work by themselves normally ive re grounded them already and have replaced all the switches ( brake turn and head light ) and it makes no difference nothing works like its suppoed to