To avoid going to "hi-tech" fixes, you could run the grids with a HEAVY relay, like maybe an external starter relay, that you control with a switch, ... I don't recall the draw of these grids but would estimate at least 50 amps, likely more. There may already be an OEM relay in the system, which would explain the flicker you're getting.
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1993 D350 DRW, Getrag G350, 760,000 km, CB, Sirius Sat radio, flat deck with headache rack.