I made non-taxable investments in a 401K from 1979 to 1982. Later, I rolled it to a trad IRA. I took my 1st withdrawal in 2018. I never filed a form 8606. Is it too late?

See the following answer explicitly addressing this situation:

https://ttlc.intuit.com/replies/6414216

This answer is helpful but when I tried to follow the steps, I never found a place to enter an explanation. Did I take a wrong turn somewhere? EDIT: I went through the steps again and this time a place for an explanation came up. I think I'm OK now. Thanks so much!

Unless you made after tax contributions to your IRA, you don't need to worry about Form 8606, as 100% of your distributions will be taxable. 

The contributions I made prior to 1983 were after tax. So I would like to file the 8606 so I don't have to pay taxes on them again. I don't know how to file it - none of the instructions seem to apply to that situation.

Well, you should have said you made after tax contributions.  When you said non-taxable I thought you meant pre-tax contributions.

Sorry - that was the language that was on my statement from 1983.