I have an 2004 A4, quatro (1.8T). While fixing a vacuum issue, I noticed a wire that is not plugged in. This loose wire is located in the engine compartment, driver side, near the firewall

I'm trying to find out if anyone recognizes this plug and where it might go?

Symptoms are -

1. P0420 Code (O2 sensor)
2. Non-working backup lights
3. Proper thermostat reading but little heat from the heater

I've attached a picture.

 

I'd like to add that the plug itself is orange, wires are blue and black with blue strip.

Any ideas?

 

That should be your reverse light switch plug connector. Look around transmission , there should be a switch it plugs into.

 

Any clue as to the location of the reverse light switch you mentioned?

Thanks,
Steve