100% this is the Power distribution module........the fuse box to you and I.
This is a common issue and relates to the grounding path you mentioned, being corroded deep within the fuse box.
I did all you mentioned above and used a switched ground flying lead in my cab, to get the running lights working.
I eventually traced out all the gound and input paths to the running lights and discovered this fuse box problem.
Do a search on here and you will find several articles on this very subject.
I personally PM'd several members on here who were somewhat scepticle about the fuse box fix. However, when they changed them out, they all worked as has mine for over 3 years now.
This is still a $250+ part, but it will sort your problems for you.
Hope this helps and has saved you a lot of grief???
Last edited by abarmby; 08-27-2009 at 03:56 AM.