I know this is a hard one to describe in writing, but I'm try to figure out if my rarely-used rear window washer is spraying properly.

There are two properly-spaced streams, but they seem to more run down the rear windshield than shoot onto the glass, if you know what I mean. The front works fine. Any thoughts?

 

Mine sort of do the same thing, Lexus must just want fluid there for the wiper to use to clean glass, means the first part of the wipe may be dry depending on conditons.

where's the reservoir for the rear sprayer?

 

The rear sprayer uses the same reservoir as the front sprayer. Uses a very long tube. You can see it if you disassemble enough of the cargo area(if I remember correctly, it's on the passenger side not too far from the amplifier(at least that's what I was in the process of getting to when I found it).