Siri volume is 0 when in driving focus in car, but works when earbuds in

For some reason, when driving focus is enabled, and Siri is connected to my car’s Bluetooth, Siri is not audible. For the longest time I thought that driving focus was busted entirely because the screen would light up, the Siri animation would flash for a while, but would not do anything. I eventually figured out that it was responding, asking for a follow up yes or no or the like, but since I wasn’t giving it a confirmation, it would just time out. Volume on maps, Waze, music is fine, just Siri. Pressing volume up on either the car or the iPhone does nothing while Siri is speaking. If I put in earbuds, then I can now hear Siri talk, and everything seems to work… But how do I get the volume on Siri to work with my car’s Bluetooth? It is a 10-year-old Toyota, no CarPlay, fairly dumb audio on the car (‘press talk button and say yes to confirm”)