I haven't tried with the car speakers. However, I did do it with a couple of standalone bluetooth speakers. It "worked" but not as well as I had hoped.
1). The audio level is kinda low since it's a headphone jack.
2). Because of #1, you have to crank up the volume on the HP-1, then you get a very irritating hiss after every transmission. It only lasts a couple seconds or so, right after the transmission before the BT speaker shuts audio back off.
It (mine) does this with any amplified speaker. Whether I use BT or an AUX cord.
Jeff.
As a side note, the bluetooth speakers work great with my 396xt. I had to drop the volume level (of the 396) down somewhere below half-way or it audio gets distorted.