Do you know why these 2 pins in this autoharness are connected?

Maybe so that one blue wire can fire up a separate amplifier and put up an antenna if that was a thing The amplifier could be a thing the antenna we don't have unless our antenna uses power for some sort of an amplifier for the antenna signal which is possible so I wouldn't mess with that that should be okay some cheaper m e t r a stereo harnesses I believe are similar that shouldn't really cause you any strife I guess if one wanted to you could cut that loop and tap onto that say if you had a power amp but our antenna is in fact non amplified then again it probably won't go anywhere on the car side if it doesn't need to so with our antenna is in fact amplified it would need the power. If it's not then it shouldn't need the power because it doesn't lower and raise electronically. If you don't have an outboard amplifier from your existing stereo like with the JBL system then that blue power wire for the amplifier probably does nothing in this situation so with this harness I guess you could add an amplifier and have a place to tap in to pick up power when you put the stereo on I remember a little bit of this just from installing stereos on various vehicles from the '70s through the late '90s buying devices to make your steering wheel buttons work the value to that can only be set by you and your mind to me it wouldn't be worth it I don't ever use the steering wheel buttons like never I will never stop reaching for a knob to turn up the volume even my stereos at home all have volume knobs even if they have a remote I rarely use the remote the knob for the integrated amplifier or whatever is right near my hand what I'm sitting My stereo is not hidden in the closet 30 ft away I'd rather not hunt for remotes. Driving doesn't take so much of my attention that I can't turn the volume up when it does then I probably won't be listening to the stereo that much. I have turned the air conditioning on once or twice in my time with the Prius via the steering wheel buttons but then it's in auto mode and I don't like it in that mode so I have to reach over to the MFD to put it in low manual medium fan.

 

That Pioneer deck won't accept steering wheel controls from a Prius unless you use an adapter box in the middle.

I'm sure there is a Maestro box to do that, but there is probably a cheaper Axxess box available.

The 2015 Prius has both vehicle speed pulse and parking brake status wires. I don't know which ones they are in yours- our car is a different model.

Regarding your wiring harness- are you saying that the power amp output from the Pioneer is wired to the antenna power line on the car harness? That may be appropriate; many Toyotas require a 12v feed from the head unit to operate an RF pre-amp closer to the actual radio antenna. When using an aftermarket head unit, you generally use the "blue power amp wire" to feed that circuit.

 

I've heard some head units had some ability to work directly, but I haven't investigated lately. It's been 6 years since I bought a new head unit. Very good news if it's true!