It's the momentary switch in the shift lever, or a harness wire has parted. I think, for the 2003, you have to replace the entire lever. You can try to "exercise" the switch (peck it quickly for several minutes), but it's usually only temporary, if it works (supposedly "cleans" the contacts). If you replace the lever, have a new lever boot handy. If original or old, it will likely disintegrate when you mess with it.