I have the wiring harness from a 2007 Grand Prix GT here in excellent uncut condition.
Complete from where it plugs into the fuse box to rear o2 connector, pcm connectors, etc...
Should be a direct fit for 2004+ Grand Prix GTP and GT with the L32 and no tap-shift.
Also, a clean alternative to hacking up an NA harness and splicing wires for top-swapping.
$150 shipped in the lower 48 states.