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In the bottom of the pan near the rear. This assumes the car will be on inclined ramps or on stands at the front when you need to drain the fluid. If the car will be level, then it doesn't matter...except for being at the bottom of the pan.
I installed mine at the front of the pan, centered from left to right. At the back of the pan, there was not enough clearance inside the pan to fit the nut on the drain fitting. The valve body is too close.
You can place it in the bottom of the pan, or you can place it on the side or back of the pan. You just need to make sure that there is clearance for the inside portion of that plug wherever you place it.
You don't need for it to drain ALL of the oil in the pan; you just need it to drain MOST of the oil so that you can drop the pan without spilling any of the residual oil.
If you don't want the plug to show when someone looks under the car, install it on the backside of the pan--but not so low that the plug has to seal on the bottom radius.