It's should work from the main java too, but see this:
stock_parts_list_F = new int[4];
stock_parts_list_F[ 0] = cars.racers.Nissan_Skyline_GTR_R34:0x0000010Fr; // "F bumper" //
stock_parts_list_F[ 1] = cars.racers.Nissan_Skyline_GTR_R34:0x0000010Dr; // "hood" //
stock_parts_list_F[ 2] = cars.racers.Nissan_Skyline_GTR_R34:0x0000010Br; // "F windshield" //
stock_parts_list_F[ 2] = cars.racers.Nissan_Skyline_GTR_R34:0x0000010Fr; // "F bumper lip" //
Are you have them in this way ? with the same ID's
The name after the ID doesn't matter, it's just for you. But ID need to be the proper one
Open the cfg of the lip, there is Resource ID line. Copy/paste the ID from there