Paint the part with liquid cement (I use Tamiya Extra Thin Cement) and then press wrinkled plastic food wrap into it. Press and move the plastic around in a random pattern. Then pull the food wrap off and what is left a wrinkled texture in the seat. Once dry, lightly sand it to remove any high spots. Paint and dry brush to enhance the wrinkle texture.
To protect areas you don't want 'wrinkled', mask with Tamiya masking tape and make sure its pressed down firmly so the cement doesn't get under it.
I got the idea from a home improvement show where they were applying paint with rags to give the wall a wrinkled cloth effect.