changed ecm, now wont start! HELP!!

[hasher]

i just joined this forum for some help(sounds like smart people on here)!
we are just a small repair shop and not to familular with the chev duramax.
we had a guy towed in with an 05 chev 2500 crewcab, 6.6 lly ,auto.
the ecm and other parts under the front right of the pickup were smashed. we replaced all the parts and the ecm from another 05 chev 6.6l, but now the truck cranks over but will not start. the security light on the dash flashes. i was told to change the big black box that is under the steering colum(elec chas cntrl mod) with one from the same truck the ecm came from (wrecking yard), so we did. and still wont fire!
can u guys help?
we dont have a local gm dealer. (in the boonies, Canada)
how do we get it to re learn the ecm?
please use lamans terms and spasifics please:)
thanks
greg
gt auto

[Kennedy]

The ECM should first be programmed to the VIN of the vehicle that it is going in. This matches equipment etc and generally the Passlock anti theft will function properly.

Otherwise, there is a manual Relearn procedure, but again what you have done is not quite proper. You could have errant speedometer, fuel gauge, odd function in climate control, incorrect emissions group, etc.