I advise against air fresheners, but 2 AAs is 3V compared to USB 5V. Find out what the draw is - I suspect a resistor (else 3 diodes) will be ok.
And yes, that is what a relay id for. Block thru fuse to relay 30 & out 87 to the lights.
Then either (say) block thru fuse thru switch to relay 86 and GND to 85, or 30 to 86 & 85 thru switch to GND.