Ok ... stop thinking "2016" backdoor anything. On the 2016 return you cannot report any conversion at this late date if it was NOT done in 2016. What you need to do is report the non deductible contribution on the 8606 ASAP.
If the 2016 contribution is still in the IRA (and hopefully has been making money all these years) then you can make the conversion at any time now however be aware that the total conversion will not all be tax free ... any earnings converted will be taxable.
