Am I missing a relay?

unfortunately no, the battery case is waterproof. the water problem is very common, and will eventually rust out the body.

the water often enters under the rain gutter trim. cracks can be sealed with silicone.

it can also enter under the hatch gasket or hinges. and even sometimes around the tail lights.

 

you can see a small rubber plug at the bottom of the battery well toward the front of the car. Pop that rubber plug out and the battery well will stay drained. Will likely be similar on the other side or the car also, under the little plastic storage cubby.

 

Glad you resolved the problem. Also check where your water leak by the battery is coming from.

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