Drilling drain holes in trunk area?

There should already be a drain in the spare tire well, plugged with a rubber plug. Just pull that for now.

Couldn't say about the battery area.

 

The battery area and the opposite area on the driver side of the car run your finger around You will find two plugs that need to be pushed through You do that then if you take your garden hose or a small bucket and pour some water in that battery area You see where water pools I use a little 1/4-in short drill bit just to poke holes where the water pools you'll see it run through your holes that you just put there's really nothing under there but I wouldn't be running a 6-in drill bit all the way through there just a little shorty drill bit or even better if you like a punch and a hammer you'd need to kind of a long punch but that will do very well even makes an indention around the hole and helps the water run to it like a drain but generally speaking no unless you're sinking something real long through there and even then I doubt you're really hit anything worth discussing You can go over and put the same holes you put in the battery well underneath the tool pouch or bucket and that will stop all the water from pooling and freezing around your battery your tools and your hybrid battery area in the spare tire well you want to push the plug through that's closest to the battery the hybrid battery so it's the one that's closest to the front of the car when looking at the spare tire well push that through It falls on the ground retrieve it and throw it in the trunk save it for I don't know what but you have it and this will allow water pass through and not water standing ideally you want water to pass through your car chassis not standing it and become moldy musty and create a terrarium. Our volt which is a 2012 volt that was given to us with $35,000 mi on it when we pulled the plugs out of the bottoms of the doors We got about 3 gallons almost out of one door Just running out of the drain hole The other three doors maybe a gallon total out of the three when we stop the car we could hear water sloshing in the driver's door It's the only way we knew it was there We open the doors pulled the plugs to drain two plugs in each door and the water just ran until it didn't.

 

The simple answer is YES, it is safe to drill a 1/4 inch hole in the bottom of those wells. I've done it on a couple cars. There's nothing but space under there. The easier way is to just take out the rubber plug located in each well. The side wells have a plug near the front.

 

I wouldn't drill holes because your creating another problem - Rust!!!!! Besides, small holes will probably get plugged up with debris anyways and you may be unlucky enough to drill through and hit something vital. Find the rubber plugs and remove them is the best course of action IMHO.

 

Not really anything to hit and a quarter inch hole does pretty well and once they're there if you live in a problematic area where there's lots of water trying to pass then you'll take a piece of wire and undo those drain holes when you need to and the larger ones with the rubber plugs in them generally won't feel up unless your car is just really nasty or something. I had a lady spill several ounces of Tide liquid blue detergent for clothes in 4 months or a little less it ate out the complete trunk spare tire well floor literally ate the steel away no kidding.it pooled in spare area plugs still in that floor

 

When the OP goes to fix this consult this thread:

Prius trunk LEAKS pictures compilation - common problem areas. prius 2004-2015 | PriusChat