I can't tell which side, but from the looks of it, its on the drivers side, BOTTOM. Item #10.

RADIATOR ASM. Fits: Aurora, Bonneville | Nalley Buick GMC Brunswick

Well, after continued searching I finally found the drain plug. It'* on the driver side at the bottom of the radiator. You need to remove the complete splash pan and then remove the plastic brace that goes from side to side just above the lower bumper valance. I removed the 2 plastic push pins connecting the brace on the driver side. It'* pliable so just move it out of the way. You will need a 19mm closed end wrench to open the drain plug. I completely removed it and put a funnel up to the drain w/ a hose on it to keep the mess to a minimum.

I knew there had to be one somewhere. Good job Bill.

So that'* where that rascal sits. Nice find Bill!. I've been scratching my head for years. When it came time to change the Dex out I had a friend with a machine change it out.