Look for Brad Paisley, Randy Travis and Carrie Underwood to take part on American Idol this week for Grand Ole Opry week.  Randy will mentor the remaining eleven contestants Tuesday night, then perform a duet with Carrie of his hit, and her current, single "I Told You So", which was written by Travis.   His version reached No. 1 on the country charts in 1988. Underwood's solo version is her current single. A new studio duet version of "I Told You So" will be available digitally the next day. Brad will perform his new single, "Then," for the first time on national television. The single is from his forthcoming album, American Saturday Night.  Paisley, Travis and Underwood are all members of the Opry.