So I was curious if there was some hack to set my headlights to automatically turn off when I take the key out and I found (don't know how to embed youtube vids). I'm not a "car guy", but I think this seems like an easy modification? What would be the correct part I need to buy to do this?

 

At your local auto parts store, ask for a 12 volt horn relay. They should have it.

 

While a good idea you need to think it through before you install it. Granted it is easily reversible.

Some questions are:

Do you want to be able to turn on the headlights with the key off.

Do you want to turn off the taillights too?

How about the parking lights?

When you have answers these questions you can then decide where to interrupt the circuit or circuits when the key is off.

Here is another approach that we have used for years. Wire a polarity sensitive alarm across the taillights and the ignition circuit. When both are turned on the alarm will not sound. When the lights are on without the ignition on, the alarm will sound. When the ignition is on without the lights on, the polarity is reversed and the alarm will not sound.

All it takes is connecting two wires to the correct locations. Unfortunately I do not own a 2nd gen anymore so I can not tell you exactly where they should be connected.

Note:: instead of connecting to the ignition, that lead could also be connected to something that is powered in accessory mode like the heater fan.

I have one of these on my motorhome and my airplane. All the rest of my vehicles have a BCM computer already turning off the lights for me so I do not need it.

I like using SonAlerts for the alarm but any Radio Shack (if you can still find one) alarm would work.

Here is one at random off of ebay. Just make sure that the one you get runs at 12 volts DC. This one runs at 10 to 40 VDC.

https://www.ebay.com/itm/Sonalert-Ma....c100005.m1851


This was not to detract from your suggestion. This is just an alternate idea.

 

My goal is simply to not have to bother turning my headlights off when I get somewhere, plus I live in the city and a lot of times I forget to turn them on because the streetlights are bright enough.

So yes, I would want the taillights on/off as well. Same for the parking lights since they run at the same time as the heads & tails.

Side note: would this method cause the hazard lights to only work with the key in the ignition?

 

I do not have a schematic for your year Fit, But my initial reaction is your hazard lights ought to still work. The hazard lights usually use the turn signal circuit or brake light circuit or a combination of both. You are asking to shut off the tail lights and head lights which are different circuits. I am at a real disadvantage trying to advise you because I do not have access to the actual wiring diagrams to look at.