I don't know about US spec, but on my UK '93 +8 the RH headlamps are grounded through a black/orange cable that goes to the dim-dip unit behind the dashboard. I don't know if your car has dim-dip, but if not, it may be that this wire is grounded somewhere behind the dash. (The dim-dip unit is a UK device that connects both left and right headlamps in series to give a dim beam when the sidelights are on).
Hope this helps