just got a 2010 Prius - need advice

I have no idea, never need to access it. But I "think" someone said you can remove the
plastic wheel lining to access it.

 

Corollas from this era use a belt-drive water pump (as do gen2 Prius)

 

The water pump isn't too bad to get to. A little tricky from above, or you ca remove a wheel and fender liner and get at it from below. Maybe if you are doing the washer pump at the same time that would make sense...

 

Yes but overheating is not a problem on gen3s unless the hg leak has been going on so long there is no coolant left. Most hg fails don’t show a low reservoir and don’t overheat before the severe rattling starts.

 

As long as 'overheating' means "the coolant temperature being hot enough at the cylinder head water outlet where the sensor is to make the red dash light come on", then it doesn't happen a lot in gen 3s.

Toyota released a TSB in Europe that comes right out and says they updated the programming for water pump speeds, to avoid having a too-low flow rate in some operating conditions lead to excessive temperatures in some regions of the engine but not others (or, excessive temperature differences between engine regions). Readings we might see from the one sensor at the cylinder head outlet on a production engine aren't enough for us to show that is or isn't happening.