There's not one code to make a hybrid in GT2.
There's what's called a basecode for each car part (engine/weight/tire/turbo, etc.)
Then there's a separate part code for each part from each car (S2000 GT1 engine, GT40 Super Soft Tires, etc.)
you just take the basecode you want for the version of GT2 that you have (1.0, 1.1, 1.2, and maybe Greatest Hits), put it with the correct part code, and there ya go.
If you want more details, you can look at my site, I think it's got that information on it. Just click the link in my profile.