If you go OEM you'll have to wire it up properly or get the conversion cable for it and tear out the car to wire it through....(yep, been there, done that). Then you'll need to trim pieces for the trunk if you want a total OEM look. If I had to do it again, I would have saved my $$ and time and bought a stick on nav unit.
'05 GMC Sierra SLT Z71 CCSB 4x4
'00 740i Sport