This is pretty straightforward to do. You'll create a profile that is triggered by the
Received Text event.
Call a task that includes a Send SMS action block. Within that, use %SMSRF in the Number field (this is the number of the sender of the last text message that came in). Type your message in the Message field. You can use the If option within the block to set the condition (such as, connected to your car's Bluetooth), or if you have some complex condition, you might want to use an If/Else/Endif structure within the task.