I am not sure if that is exactly how the mirror is. I know I took mine off because I installed a dash cam behind it, and mine actually came with the automatic high beams, so there was originally a camera there. But it only works with auto headlights on, which I never do (just a weird preference) so I tore that out and put my dash cam in it's place. It's hidden and looks OEM and love it. Back to the topic - when I took the mirror off it was just a big bolt and I am pretty sure we just turned the mirror to the left and it unlocked from it (kind of like a light bulb) and went on the same way. You should be able to find a mirror and just bold it up IF the bolt is still on the window. I know my friend's Dart had the mirror fall and the locking head in the window also was gone. Not sure if that is what situation you are also in or not, but sounds like you still have it there. Here are some pics from when I did that
If your son's Dart is an SXT, it might not have come with auto dimming or any other features in the mirror. As you can see in my pics, that white block by the bolt (not my ez pass) is the harness. However, I had a lot of stuff in my mirror. Mics for BT, auto dimming, auto high beams, my auto wipers sensor is near there too. So obviously I have a harness, but some lower end SXTs didn't come with BT, handsfree, or auto dimming. If it has BT, then something happened and they took out the harness with it, but it's possible it was just a basic one.
Yeah the lower modes did come with those style keys, but I don't think the SXT did. As I continue reading, it sounds a lot more like this is an SE, (which only came in 2.0) but I will just assume someone got the spare for it. Even SE's came with the fob but some LOW LOW LOW level SEs came with only the key. Those are really rare tbh unless ordered that way. A very active member on here I believe uses that style in his SE's. Regardless, yes they are way too expensive at the dealer. I believe you can buy a pair on ebay or something and find an advanced locksmith that might have the software to do it. Dealer will say it can't be done, they have to do it, but they are full of it - as always.
Glad the major issue is only suspecting it's been worked on. It's possible it had an engine problem and needed to be replaced. Which is rare for the 2.0. The 2.4s...not so rare. lol But that auto transmission is also very solid. Much much better off than the manual (which I own) and the DDCT. The DDCT which has been coming up a lot more frequently here with MAJOR repair bill stories because now all the Darts are starting to get out the 5 year powertrain warranty. People are finding out this new "tech" that everyone seems to be putting in the new cars and saying they are stronger, shift faster, more reliable are now dumping the cost of a NEW transmission in repairs just to replace the clutches. Hell the dealers don't even know how to work on them as many people have been told they are not serviceable, which is completely false.
A word of advice for your dart. Spart plugs on the 2.0 are due at 100k but I would do it before that. Irridium NGK or Champions is what they use, I believe the NGKs are better. I would honestly just do a whole tune up on it if it's reaching that mileage or at least check them. It's stupid simple on this car - right on top. The Trans fluid is due every 50k miles. This goes against what is said in the book as it says it's a sealed transmission and no maintenance is needed. LOL yeah then when it blows up they charge you for a new transmission... Anyways, I have a master tech friend in the Mopar dealers and he said this is the new protocol and it should be changed every 50k. They might have updated in the newer manuals, but not sure. So I would probably get that done as it was not. There are a few threads on here regarding the procedure. Ummm check the wheel bearings and struts. People tend to go through those often on here. check your Strut tower for any rust under the caps in the engine bay. For some reason the 13s were bad with this. Coolant is OAT so it's specific and can't just add any green stuff too it. Prestone does make an OAT coolant that meets Chrysler MS number but I cannot think of it off the top of my head. There is a big big thread about that that Youngsmith made, or maybe in his giga post with all the info he made a section about it. Keep a look out for his old posts, very informative but I haven't seen him in a long time.
Other than that keep on driving it and good luck with it!