Bought a used car and noticed it has a jack in the dash. Bonus, I can listen to my jams through the car stereo! It is a male jack, but it isn’t 3.5 mm it is smaller. Okay, did a quick google search and see that some older devices had 2.5 mm jacks. I bought a adapter but it still doesn’t fit, the male pin is too large to go into the adapter.
Um…so what might this jack be?
Is it the manual eject for the CD player? 2.5mm is the smallest I’ve heard of.
Are you sure it isn’t a reset button or something like that. Some of those are the size where you need a toothpick or paperclip to press it.
year/make/model of car, please.
I must not have been clear, it is an aftermarket set up to allow you to access a mp3 player. So the jack is the male end, just like the end of your headphones you plug into your mp3 player … except, you know, a weird size.
No, you weren’t clear. You said, “It has a jack in the dash.” Who told you that jack is for an aftermarket MP3 player?
you’re still not clear, because a “jack” is a receptacle, and the male end is a “plug.”
Sorry, I thought that saying it was the male end would have made it clear. Anything that could be mistaken for a paperclip reset would be female would it not?
This is a male jack on about 4 inches of wire below the center of the dash so that anything connected there could rest in the pocket forward of the gear shift.
And you can tell it isn’t OEM. Yes it is an assumption that it is for a mp3 player. What else might it be?
So it sticks out of the dash, then? It’s not a jack, it’s a plug. If it was meant for an external audio device, like an mp3 player or a portable CD player, it would be female, not male, and take a cord with a 3.5 mm pin on each end.
Without inspecting the car and tracing the wiring there is no way to know what it is. We can guess, but that’s no better than your guesses.
If the connector is not the typical size or shape for audio, it might be a power connector to supply power to something. Such as a radar detector. Many devices over the years have used all sorts of novel plugs & sockets for that.
For damn sure anything that has a male plug on a pigtail is non-standard. Which means the possibilities for this thing are as varied as the crazy clueless bastards that wire things in non-standard fashions can think of.
How old is the car? That would at least set the era. e.g. if the car is from the 1970s we might expect a power connection for an off-board 8-track. if it’s a 2010, not so much concern for those.
Any number of things. A radar detector, a CB radio, a police scanner, a microphone that runs to the speakers that you still don’t know are hidden inside your front fenders. 
Hell, I dunno. But if it’s not a normal size for an MP3 player, then I’d be pretty sure that it’s not one.
2004
Okay, vocabulary expanded. It is a plug.
But wouldn’t the easiest connection to an external audio device be a plug that would go into the earphone jack of the device?
This… I am taking to heart.
No, because those cables break all the time (rear passengers want to plug in their player? hand it over and… oops) and it would suck having to rewire your car every time your stereo cable broke.
Do you have a cell phone? Take a picture of it, maybe with your finger next to it for scale.
Yes, but the stereo would have a similar jack in the front of it and be connected with a cord with a plug on each end. You wouldn’t have a cord with a plug sticking out from under the dash.
My LG cell phone has a socket for an earpiece with speaker and mic that takes a plug significantly smaller than the one used by iphones.
I have seen devices that took power through a plug that looked a lot like a earphone plug, but smaller.
There are any number of things that plug might be. Before I plugged it into anything valuable, I’d want to crawl under the dashboard and see where the wires go at least.
Well, I have had a car with a connection for an external audio device (CD player) that was just a plug coming out of the dash. The radio had two RCA jacks as inputs, and I had bought a RCA-to-3.5mm cord and attached it, then stuffed most of the cord up into the dash so the only thing coming out was a few inches of cord ending in a 3.5mm plug.
Can you at least get the brand and model number of the head unit? Or is this ‘thing’ not in the head unit but somewhere else? If it is in the head unit (radio/cd player/whatever), then maybe it could be looked up online.
It might be a 3.5 mm plug after all. They come in surprisingly variable sizes; the OEM jack in the dashboard of my last Hyundai would accept any 3.5 mm plug but none of them fit quite right and I would only get mono audio output unless the plug was seated “just so.”