AN AIRBUS FLC WAS ON THE WRONG FREQ WHEN DOWNWIND FOR THE MSP ARPT. DURING THIS TIME A TCASII TA WAS RECEIVED FROM THE PRECEDING ACFT.

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AT 7000 FT ON APCH WE WERE BEING VECTORED TO A DOWNWIND BEHIND ACFT Y; A B737. THE CTLR TOLD US TO KEEP THE B737 IN SIGHT AND THAT WE WOULD BE FOLLOWING HIM FOR THE VISUAL APCH TO RWY 12R. HE THEN GAVE US A FREQ CHANGE THAT I MISTAKENLY THOUGHT WAS THE TWR. WE BOTH COMMENTED THAT THIS SEEMED EARLY TO SWITCH TO THE TWR FREQ. WE CONTINUED BEHIND (ON DSNDING TO 5000 FT ON DOWNWIND). WE CALLED THE MSP TWR WITH NO RESPONSE UNTIL THE SECOND CALL. THAT IS WHEN THE TWR TOLD US WE WERE SUPPOSED TO BE ON 126.95 INSTEAD OF 126.7. WE WENT BACK TO 126.95 AND WERE TOLD TO STAY AT 5000 FT AND A HDG OF 300 DEGS FOR VECTORS TO FINAL. WE TOLD THE CTLR THAT WE HAD MISTAKENLY SWITCHED TO TWR FREQ BECAUSE WE THOUGHT THAT IS WHAT THE LAST CTLR ASSIGNED US ALONG WITH CLRNC FOR THE VISUAL APCH. HIS COMMENTS WERE THAT HE WONDERED WHERE WE WERE AND THAT THERE WAS NO PROB. WE WERE BEING VECTORED A LITTLE DIFFERENTLY THAN NORMAL FOR SPACING. JUST BEFORE WE GOT THE FREQ CHANGE; OUR TCASII SAID 'TFC; TFC.' IT WAS THE ACFT WE WERE BEING VECTORED BEHIND. THIS EVENT SHOWS HOW HUMAN ERROR FOR WHATEVER REASON CAN BEGIN THE CHAIN OF EVENTS THAT NEEDS TO BE BROKEN.