A witness in the corruption trial of Senator Ted Stevens of Alaska has told the judge he never would have testified if he had known he did not have immunity from prosecution. The witness, David Anderson, said in a Dec. 15 letter that he wanted ''to be perfectly clear'' that he would have invoked his Fifth Amendment protection against self-incrimination without such a deal. The letter from Mr. Anderson, who oversaw the start of renovations on Mr. Stevens's home in Alaska, was made public in papers from Mr. Stevens's lawyers, who are seeking to have the senator's conviction overturned or to win a new trial. Mr. Anderson said he thought he had an immunity agreement with the Justice Department, even though he testified during the trial that he did not have such a deal. The disclosure follows the release of another letter on Monday, from an F.B.I. agent, raising complaints about the Justice Department's conduct in the trial. [ABSTRACT FROM PUBLISHER]