Will an AVIC-D3 fit in a 2004 Pontiac Grand Prix GT2 w/o monsoon system?

If so what is required for installation ie: harnesses, interfaces, antennas

For your car Yes, it will fit.
you will need 3 items to install it correctly:
A Dash Kit
Wiring Harness
FM Antenna Adapter.
As far as where to get them, you can get the parts at [...]