it's not impossible to swap automotive for computing...
(I knew a guy who started out as a tool maker at MG (before the days of robot assembly lines), when I met him he was writing software...
that said... it's not an easy switch, there are little similarities, and you're not going to get paid the same kind of wedge being a starting out (< 1 year experience techie) as you'll have got as a > 20 year experience mechanic...
I'd consider school as a waste of money in this case (which I say entirely because of your age) if you were 18, fresh out of school saying I really want to work in computers, I'd say go spend 3 years at school getting a CS degree. (because 20 years down the line that will open up opportunities that you wouldn't have as a non-graduate.)
But you're saying you're 54 and already sank a bunch of money into a failed venture, and I get the feeling that you're coming at this from more of a want to get away from the garage thing rather than love computers and always dreamed about a job trying to figure out the inventive ways that computers seem to die...?
also, lets imagine that in an ideal world you want to retire at 65, going off to college for 3 years full time is not only expensive, it's a quarter of the time that you want to do the job before retiring.
and I'm assuming you can't just announce to the wife (assuming you're married) "I'm off to school love, you'll have to cover the house for a bit" then you'll be part time student with a part time burger flipping job for five years? (almost half the time before you want to retire?)
lots of things you can study online for, (there is a bunch of A+ sites out there), and people here are bound to answer and help with questions that you may have.
after you've done the A+, then you might want to look at some vendor specific certifications (i.e Microsoft certs, or redhat certs, Cisco etc).
However, when considering this career switch, think of this:
when I walked into my first computer tech job, (almost 15 years ago), it was literally a walk in, the company had been informally looking for a techie guy to ease the managers workload (who as well as managing staff and writing software was trying to set up all the infrastructure as well.
my interview (if you could even call it that) was a pleasant and informal discussion, where I revealed that my computer experience was we use them in school (this was about 1 week after leaving school) they revealed a not magnificent wage, (which was better than anything else that I could get!) they said see you Monday, and that was that...
(they let me be part time whilst I was studying through university) - part time meaning in the holidays I came back and worked long days and caught up the problems they had, and setup all the infrastructure for new upcoming projects.
When I left the company five years after I first started, they received about 20 CVs, held interviews and set written tests for the job.
I started at the job I have now about 7 and a half years ago, there was a phone interview and two face to face interviews for a first line position, (i.e call answering and password resetting)
I've progressed within the company through 1st, 2nd and 3rd line support, and now work deploying infrastructure.
but there are no 1st line jobs in this country within my company any more, they have all been sent overseas to places where it is cheaper to fill seats. (this means that unless you fancied a move to eastern Europe then as someone new to the arena as it were, there is no way into the company I work for to you, - not that you're saying you want to work where I work!)
the crux of that little story is don't underestimate how competitive a job in the IT industry can be. or how difficult and financially painful a late stage career change can be!
that said, you do only live once, so best of luck!