We present detailed multi-wavelength observations of GRB 161219B at $z=0.1475$, spanning the radio to X-ray regimes, and the first ALMA light curve of a GRB afterglow. The cm- and mm-band observations before $8.5$ d require emission in excess of that produced by the afterglow forward shock (FS). These data are consistent with radiation from a refreshed reverse shock (RS) produced by the injection of energy into the FS, signatures of which are also present in the X-ray and optical light curves. We infer a constant-density circumburst environment with an extremely low density, $n_0approx 3times10^{-4}$ cm$^{-3}$ and show that this is a characteristic of all strong RS detections to date. The VLA observations exhibit unexpected rapid variability on $sim$ minute timescales, indicative of strong interstellar scintillation. The X-ray, ALMA, and VLA observations together constrain the jet break time, $t_{rm jet}approx32$ day, yielding a wide jet opening angle of $theta_{rm jet}approx13^{circ}$, implying beaming corrected $gamma$-ray and kinetic energies of $E_{gamma}approx4.9times10^{48}$ erg and $E_{rm K}approx1.3times10^{50}$ erg, respectively. Comparing the RS and FS emission, we show that the ejecta are only weakly magnetized, with relative magnetization, $R_{rm B}approx1$, compared to the FS. These direct, multi-frequency measurements of a refreshed RS spanning the optical to radio bands highlight the impact of radio and millimeter data in probing the production and nature of GRB jets.
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