I don't know if this helps, but I run my HP2 from the line out Jack to the input on my trucks factory stereo auxiliary input.
I have noticed that this output is still active even if you push the mute button on the HP2 screen, except for the key clicks and such, all radio transmission audio comes through.
I think this is the only way to have a remote speaker for the unit. It does have a built in speaker that works fine in low noise environments.