You do not need the DD-12 discharge document but you do need the veteran's service branch, rank, and at least one date during their service (e.g., their starting date at the rank) and you need the SSN, full name. If you can't find that information you could guess, I suppose. But you can also get the government to find the DD-12 with that info on it for you... but they warn it could take up to 90 days and that they get over 4,000 requests a day for finding this kind of info.
Once you have it go here https://www.bmwusa.com/contact/military-discount.html
You don't need to build out a vehicle; just look for the panel with the ID.Me button on it. You click that and the process starts. It takes 2 minutes. See the pictures below.
You need to get an ID.Me login from the government... this is what you can also use for your e-file tax returns.
For me today, it was a way to remember my Dad again and appreciate one more gift he gave me. I just wish I could take him for a ride and talk some more.