Is this the yd25 engine?
In my 07 d40 STX the coolant drain bolt in the engine is in the left hand side of the block
If you look into the engine bay at 90 degrees directly above the front left tyre it's a smallish, i think 14mm bolt with green paint on it
Mine was a complete prick, took a 600mm 1/2 inch extender bar to get onto it, was super duper tight so even with gently applied force from an electronic impact driver it rounded the head of the bolt. Tried a bolt extractor and destroyed it even more
Took it to the mechanic and he said leave it in
Too much plastic and rubber around to apply much heat, and the block can crack if you try to brute force it out
I just have to drain as much out as i can now every 10 thou and top it up with best coolant i can get
Proceed with caution...