  An 11-day monitoring campaign in late 2005 reveals clear correlation in
polarization between the optical emission and the region of the intensity peak
(the "pseudocore") at the upstream end of the jet in 43 GHz Very Long Baseline
Array images in the highly variable quasar PKS 0420-014. The electric-vector
position angle (EVPA) of the pseudocore rotated by about 80 degrees in four
VLBA observations over a period of nine days, matching the trend of the optical
EVPA. In addition, the 43 GHz EVPAs agree well with the optical values when we
correct the former for Faraday rotation. Fluctuations in the polarization at
both wavebands are consistent with the variable emission arising from a
standing conical shock wave that compresses magnetically turbulent plasma in
the ambient jet. The volume of the variable component is the same at both
wavebands, although only ~20% of the total 43 GHz emission arises from this
site. The remainder of the 43 GHz flux density must originate in a separate
region with very low polarization. If 0420-014 is a typical case, the
nonthermal optical emission from blazars originates primarily in and near the
pseudocore rather than closer to the central engine where the flow collimates
and accelerates.
