It is vital that in a Constitutional proceeding as important as a Senate Impeachment trial all evidence and testimony can be heard so a fully informed decision can be made. This amendment would allow both the prosecution and defense to calls witnesses at their own discretion as would be the case in any trial. 

Article I, Section 3, Clause 6 would read as follows:

The Senate shall have the sole Power to try all Impeachments. When sitting for that Purpose, they shall be on Oath or Affirmation. When the President of the United States is tried, the Chief Justice shall preside: And no Person shall be convicted without the Concurrence of two-thirds of the Members present. Both the Prosecution and Defense shall be allowed to call Witnesses.