Even though you hand write your FMM card it has a bar code on it and is scanned into the INM system. INM Ihas gotten serious in the last 2 years about enforcing immigration law.

I'm sure you are correct. The issue with a vehicle is that you're dealing with two separate departments, Immigration and Customs - and the question is how much information-sharing they have between them.

Actually there is a provision in place for emergencies. It involves leaving the vehicle in an Aduana facility.

Have you ever used that service? How/where would I find details?

Yes, we have done it recently. Annually for the last 35 years, through all the changes.
Until this year, because we are applying for our Permanent Residency, we have purchased the TIP on line. That way we don't have to wait in line at the border, or even stop. We also make a run to Tijuana, walk across and get our FMM. Specify 180 days. There is no connection to the vehicle at all. The FMM automatically expires when the time is up, and that's it. At this time, and who knows for how much longer, FMM's are hand written, usually by the person who wants it. It's possible, but don't see how the number and the vehicle would be connected. I thought the vehicle deposit HAD to be paid by credit card.

Sorry for being unclear. When I asked if you had "done it recently", I specifically meant have you entered with your vehicle on a 180 day tourist card, then flown out of, and back into Mexico during that period, then driven your vehicle out within the original 180 day period. I was wondering if, in that situation, there was any particular advantage to having purchased the TIP on-line, instead of at the border on entry.
Someone has noted that when you enter Mexico, the tourist card has a little block where you indicate arriving by land, sea or air. Check "land"....because you arrived with your vehicle....and you may be caught when handing-in your tourist card at the airport: "Where's your coche, Senor?"
The deposit can be paid in US cash or by credit card. We've always paid cash and have never had a problem getting it reimbursed at the border on exit. Obviously, we have to line up at the Aduana counter to get our money.....the only disadvantage.

Regardless of what the law says, one would think there would, or should, be provision within it for an emergency situation where one has to return home on very short notice.....as long as the vehicle eventually leaves Mexico within the original 180 day TIP period. But that would make too much sense, I guess.

On other details, I've found customs and immigración can be real sticklers.

We drive down every year, and have for 30+ years.
Get your vehicle permit on line. No extra cost, easier. Then, when you get to the border, you get your FMM (visa) When you fly home, you will surrender your FMM at the airport, and get a new one on the way back, included in the price of your ticket. Return your vehicle permit before 180 days at the border. They come out, remove the sticker, and you will be credited for your deposit in a day or maybe two. You do NOT need to turn in your FMM. As aduana says, "It expires."
Very straightforward, very easy.

Certainly sounds easy. I assume you've actually done it, recently?
What is the advantage (other than, perhaps, convenience) of obtaining the Temporary Import Permit (TIP) on-line? In the situation we're discussing, are you more likely to get your vehicle deposit returned because you purchased it on-line, rather than simply at the border on entry?
I've heard that there is a now a link between the registration number of an FMM tourist card, and the registration number of the TIP. Turn-in your FMM and the computer rings a little bell saying "Where's your car?" One foreigner who had to fly home during his 180 day tourist card period reports being tracked down at the airport just as his flight out of Mexico was to leave, and required to immediately pay a 390 peso fine for leaving the country without his vehicle. If that was the worst that could happen, I'd go for it. The story didn't provide any details of what happened months later, when he returned to the border with his vehicle....did he get his deposit back? Was he banned from ever bringing another vehicle into Mexico? Etc., etc. Or maybe he's in a Mexican slammer and hasn't been heard from since?

PS: We've driven down many times, so are totally familiar with the "normal" procedures. We've always paid our deposit in cash, and have never had any problem getting the deposit back when we've exited the country.

"When you leave the country the car has to exit with you"

"When a car exits Mexico the individual listed on the permit has to accompany it"

Has this changed?

Also years ago the SSP audited my car, the TIP, my driver license, and FMM. Line by line point by point. They would have caught a discrepancy between the car permit and immigration permit for sure.

If a car is conveyed to the border by tow-truck, what happens then?