I don't know the address or anything, I just have the .dat file itself from a backup (all the other info was lost) and the passphrase.
I'm willing to download the full blockchain again, that's not the issue, but I was wondering why there's no good info out there to really restore an old wallet file.
Even if you do know the address, if it's an old backup, it would be better for you to resync the whole blockchain to ensure that you'll be able to get all the coins in the addresses within your wallet.dat.
So if I have 2011.dat for example, and rename it to wallet.dat and place it in the bitcoin folder where the new wallet is then restart bt core then it should work?
I heard older wallet files were incompatible since 2012 or something.
Yes. Do keep a backup of your wallet.dat though (both your current and 2011). It's true that the wallet.dat files had its structured modified slightly but its not wholly incompatible. The client would be able to identify and upgrade your wallet.dat to ensure compatibility.