Why wont this bloody truck start

OK so we have a 2001 chevy silverado( my Husbands favorite automobile of all time evadentally)5.3l, 4wd,     4L60e transmission
Transmission messed up the middle of last year and it has been parked since,
We replaced the transmission , replaced the starter, alternator, Battery etc
Truck will not start from the key, can cross the solenoid and it cranks.
OK  everyone is gonna say Neutral safety switch on the tranny, Done changed it.
Now here is the kicker...someone explain this
It will not start from the key, all dash lights are on and reading, the  key bell is dinging away...nothing
However, if you open the hood, remove the cover of the fuse box and wiggle the Starter relay...the truck will crank....
I know...where is the short...We have checked wires , fuses, grounds etc etc and can find nothing wrong, so why is the bloody truck still not starting?
Have a good mechanic working on it, and spoke to other mechanics about it, our mechanic spoke to other mechanics, noone can figure this one out.
So any help would be appreciated