Please see this answer from IsabellaG.
You can file delinquent Forms 8606, even as far back as 1995, on a standalone basis, meaning that you can file them without amending your tax returns. There can be a penalty of $50 for not filing Form 8606 on a timely basis, but the penalty can be waived if you can show reasonable cause for not filing.
File these delinquent forms separately from your tax return.
See this IRS link regarding Form 8606 and see this information for suggestions regarding “reasonable cause.”
