I switched jobs in 2018 & over contributed to my 401(k) by $1600. I have asked for an excess reimbursement from my plan admin but how/where do I note this in 2018 taxes?

Reporting excess salary deferrals (excess 401(k) contributions) returned to you after the end of the tax year but by April 15th of the following tax year on your 1040.  Do not create your own 1099R for this situation.

Any excess 401k contributions returned before the end of the current tax year should be covered in a 1099--R from the 401k for the current tax year and enter as a 1099-R in Turbo Tax.   

Page 10 of IRS Pub 525 under Excess deferrals (the IRS term for 401(k) contribution is deferral) tells us to include the excess deferrals as income on line 1 of Form 1040 if the money was returned after the end of the tax year but by April 15th of the following tax year.  You need to report only the excess contribution, not any money generated by the investment of the excess contribution.  What you earned will be covered by a 1099-R for the following tax year and will be entered then as a normal 1099-R.

Below is how to do this in TurboTax:

• Click on Federal in the left-hand column, then on Wages & Income
• Under All Income, scroll down to Less Common Income
• Select Miscellaneous Income and click Start
• Select Other income not already reported on a Form W-2 or Form 1099 and click on Start
• Answer the question "Did you receive any other wages?  Yes
• Click through the questions till you get to Any Other Earned Income
• Answer Yes to Did you earn any other wages?
• Indicate Other as Source of Other Earned Income and click Continue
• For the description enter "2018 Excess 401K Deferrals" and click on Done