2000 W900 clearance light issues

when hooked to one trailer the parking light switch works as normal, but hooked to another trailer the parking light switch does nothing. Parking lights will only come on when headlight switch is turned on. When no trailer is hooked it is like that too (parking lights only come on with headlight). I am assuming this is a ground issue but really don't know where to even begin checking. Thought maybe someone has had a similar problem. on newer trailers I have the issue but one older trailer I have switches work as normal, don't know how that makes a difference but it does. Also truck has l.e.d. lights all over.

 

Ok Thanks guys. I'll try a new ground wire to switch 1st and see what I get.