Odd thing in my car door

I just took my door card off on my 2007 NC.

There is a little plastic box wired to the big speaker mounted on the door, containing what resembles a light bulb, a coil, and a capacitor. I have never seen such a thing in a car before, obviously it was added to the stereo. What is it and what does it do?

Thanks!

That is a passive crossover for a speaker. 

If i remember correctly coils act as low pass crossovers and capacitors act as high pass crossovers. They may be wired in parallel splitting one signal into two to feed a woofer and a tweeter (as appears in this case since 2 wires enter and 4 wires leave) or wired in series to make a 'bandpass' filter. 

Every aftermarket speaker that has a tweeter in it has one or both things, but usually they are hidden somewhat on the speaker housing. The capacitor is usually a tiny one tucked under the plastics of the tweeter housing, and if there is a coil (there usually isnt, with the woofer acting as 'full range' and the tweeter having the high pass filter) it is usually shrink-wrapped onto the outside of the woofer magnet. 

So these things are fairly common on speakers, but they aren't always prominently displayed in a separate box so that you notice them. When the speaker has a legitimately high power handling these components need to slightly larger to dissipate more heat, but usually they are only bigger to make the thing the company is selling seem fancier/worth more money, thus the clear cover. 

Yup.  Passive crossover.  I have them in a couple cars.  Most speakers have them built in (coax speakers with a separate woofer and tweeter in one basket).  High-end audio separates them out into two or more individual speakers.  The higher the frequency, the more directional it is.  The door is a great place for mids and bass, but highs should come from a source that is equidistant from each ear and closer to level with your face.  Easier to do with a 1" tweeter than a 6.5" basket.

With a coax speaker, they usually include a single capacitor that blocks low frequencies going to the tweeter.  Often called a bass blocker.  by using a complete crossover like you pictured, you can more accurately set up two distinct bandpass signals; one that separates the low frequencies and the highs to be sent to two different drivers.

Qanon told me it's how the government tracks peoplE who hoard pudding. You want to come clean about something?

PUDDING HOARDER. 

Thanks! I have never done an aftermarket stereo in a car, I've always been concerned with the performance side of things.

I heard people on the Miata forum whine about the Bose speakers in the NC, and I was like "my system sounds really good to me", didn't know the previous owner adjusted it LOL.

The filter thing makes great sense to me, I'm an electrical engineer so the moment I see a cap and an inductor together I think "filter!" but the bulb kinda threw me, maybe it is there as a big resistor for some RLC scheme. I'm guessing it is there to split the feed for the speaker and the tweeter mounted together, based on Curtis73's post.

Fueled by Caffeine said:

Qanon told me it's how the government tracks peoplE who hoard pudding. You want to come clean about something?

PUDDING HOARDER. 

I have only hoarded paper towels, toilet paper, and my hoard of cleaning wipes is down to the last 4 canisters. I did also buy 3 years worth of 0W-20 synthetic and 5W-30 synthetic when Costco had them on sale a few months ago. 

Fueled by Caffeine said:

Qanon told me it's how the government tracks peoplE who hoard pudding. You want to come clean about something?

PUDDING HOARDER. 

Want to worry about tracking?  BUY A TESLA, they track the hell out of you.