can something else be wrong but main radio head unit?

I would start by self-diagnosing the USB power issue, which may inform you on related issues.

 

Owner's manual to check for which fuse, then a $15 TIS subscription to trace the USB power and data circuits back to the CAN BUS.

 

For 2013, the factory-installed sound system in the Prius v THREE is what the sales brochure calls “Display Audio with Navigation and Entune.” I think this may be what Toyota’s Repair Manual calls the “Navigation System (Radio and Display Type),” which has a main head unit (Radio and Display Receiver Assembly) plus an Extension Module (Extension Box), a separate unit mounted out of sight, below the head unit.

From the symptoms you describe, I believe the most likely cause of the problem is a disconnected or faulty Extension Module (PT296-47141) or fault in the wire harness (PT296-47120-AB) that connects it to the head unit. The Extension Module provides the applications accessed through the APPS button, it’s involved with Bluetooth, and the USB port (No. 1 Stereo Jack Adapter Assembly) is connected directly to the Extension Module on cars that have one. (Otherwise, the USB port is connected directly to the head unit.)

It seems someone already posted the accessory installation instructions for the Extension Module for a 2012 Prius v, which you might find useful for reference, though some of the part numbers have changed. For the 2013 version of these, you may have to subscribe to Toyota’s Technical Information System (techinfo.toyota.com), which also provides the Repair Manual and Electrical Wiring Diagram. The Repair Manual gives detailed troubleshooting instructions, including procedures for checking the wire harness.

Hopefully the problem is just a bad connection, but if not, and you don’t want to buy a new Extension Module and don’t mind losing the navigation capability and other applications, it might be possible to permanently remove the Extension Module and its wire harness and connect the USB port directly to the head unit instead. I think this would give you about the same configuration as a 2013 Prius v TWO.

 

You’re most welcome. I have no experience with the Pioneer product you mentioned, and I haven’t evaluated its compatibility with the Prius v. If you decide to install an aftermarket head unit, of that style or another, I hope it gives you many years of pleasant listening.

If you’re looking for someone to say that the existing head unit has failed and has to be replaced, though, I can’t do that from the symptoms you’ve described. Before I’d give up on the existing system, I’d suggest making sure the problem isn’t a loose connector or a defective wire harness (a $30 part). You could do this troubleshooting yourself, if you’re mechanically inclined, or you could pay the Toyota dealer or an independent stereo shop to do it. Most of the work, by the way, is the disassembly and reassembly of all the trim—exactly the same steps that would be required if you were installing a new head unit.

If the Extension Module is faulty, I wouldn’t buy a new one at the list price. I’d probably just remove it, or if I found the APPS functionality valuable, I might consider buying a used one from a junkyard or eBay seller (search for 86100-YY050).

 

Yes, there is. Since the system was working before, if the wire harness is now faulty, it’s most likely due to physical damage, which should be visible in a close inspection.

Look carefully at the entire harness in good light. Check that none of the wires is broken, cut, chafed, pinched, or detached from its terminals. Check that the connector housings are not cracked or broken. Check that tabs, latches, and terminal position assurance locks, if any, are undamaged and engage properly. Check that each terminal is secured in the connector housing and not backed out, bent, or otherwise damaged.

With a multimeter, you could check the continuity of each wire, but be careful: it’s easy to damage terminals, especially smaller ones, by trying to insert a meter probe into a socket meant only for the tab of a compatible terminal, or into the space between a terminal and the connector housing. If you don’t have very small probes (like those in Toyota’s Test Lead Set) and don’t see any other damage, it might be better to skip this.

There is more information about how Toyota’s connectors and terminals work in the introductory sections of the Electrical Wiring Diagram or in Toyota’s Wire Harness Repair Manual (PDF), last updated in 2002.