After I replaced my stock HU in my sedan, I noticed that I no longer had any AM radio reception. The vehicle antenna was firmly connected to the radio, so that wasn't the problem.
Turns out that the Yaris has a powered antenna! The connection to power the antenna is in one of the "unused" positions on the aftermarket adapter harness, so you have to wire it up manually. Thanks to enkid from Yarisworld for the info! See this thread for more:
Toyota Yaris Forums - Ultimate Yaris Enthusiast Site - View Single Post - No AM radio signal on two aftermarket HUs?
Note to the admins: Can this get stickyed either here or in the electronics forum? It seems like this is not well known among installers or the public.