We describe how the combination of VLBA and optical polarization monitoring identifies the site on the VLBA images where the variable optical emission occurs. Correlation between the optical and X-ray and/or γ-ray light curves then establishes an association of the emission across the electromagnetic spectrum with a particular feature on the images. Application of this technique to blazars monitored by GLAST will allow us to construct multiwaveband emission maps of their compact relativistic jets.