i think you will find that your photos are darker with a bright background because your camera is measuring a centre weighted average of the brightness of the entire field of view. the bright white makes the camera think it has to darken the picture so that it isnt over exposed [it thinks all images average to to a medium grey, which they do not], and then the dark parts where you want detail are too dark.
there are a number of fixes for this. look and see if the camera you are using has a spot metering option, and then meter off the dark part of the field [normally done by pressing the shutter button half way], then align the picture as you want it and take the photo. if you have a manual setting, try for a small aperture [on manual] to get a long depth of field [DOF] where the front and back of the seat are in focus. to do this you will have to increase the length of the exposure, so use a trypod and the cameras timer to eliminate movement when you press the shutter. hope that helps. i think that items stand out much better on a white background, so it may be worth the hassle to get the desired impact.
