Siri audio selection...

[JUMPINTHEGUN]

When I get in my truck, my 4S connects to the BT in the truck and sometimes I also plug the 3.5mm jack into the stereo for music. The problem is that when I plug in the 3.5mm jack, Siri wants to use that connection instead of the BT connection. Is there a way to tell Siri to use the BT instead of the headphone jack? Right now, I have to make a call (it will default to BT), then Siri uses the BT connection. I cannot find an audio selection for Siri (like voicemail or phone calls). Any suggestions? Thanks.