AN ACR MLG FAILED TO TAXI WHERE IT WAS DIRECTED AT DTW IN THE FOG.

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WE WERE ISSUED TAXI OUT INSTRUCTIONS FROM GATE TO TAXI TO THE OUTER TXWY (ENRTE TO TKOF RWY 3L); HOLD SHORT OF WOLF. COMING OUT OF THE ALLEY WAY BTWN CONCOURSES THE CAPT TAXIED STRAIGHT AHEAD UNTIL WE CAME TO A TAXI LINE. WE FOLLOWED THE LINE UNTIL WE HAD COME TO WHAT WE THOUGHT WAS THE INSTRUCTED PLACE (OUTER SHORT OF WOLF). I TOLD GND EAST I WAS SWITCHING FREQS. BEFORE I WENT OVER TO NEW FREQ HE ASKED IF I HAD SEEN AN ACR FLT TAXIING AND I SAID 'NO.' ON CHK-IN ON GND WEST FREQ; WE WERE TOLD TO TURN L ON WOLF; CROSS RWY 27/9 TRANSITION TO TXWY Y AND CALL ON TXWY Y. THE CAPT FOLLOWED THE TAXI LINE AND I CALLED XING RWY 27/9. WE MADE A R TURN ON WHAT WE THOUGHT WAS A CONNECTING TXWY FROM WOLF TO Y TXWY. SOON WE DISCOVERED WE WERE ON A CONCRETE APRON WITH NO TXWY. WE INFORMED GND WE DIDN'T KNOW WHERE WE WERE AT THE SAME TIME THE CAPT WAS MAKING A 180 DEG TURN BACK EBOUND. I TOLD GND I COULD NOW SEE THE TXWY FOX SIGN. WE WERE TOLD TO HEAD BACK W AND CALL ON TXWY Y WHICH WE DID. BY THIS TIME; THE CAPT AND I FIGURED OUT WHAT WE HAD DONE. INSTEAD OF XING RWY 27/9 ON TXWY WOLF; WE WERE INSTEAD ON FOX. WE HAD THEN TURNED R ON TXWY U ON THE CONCRETE APRON N OF THE TWR. AT NO TIME DID THE CAPT OR I FEEL WE WERE CLOSE TO VIOLATING AN ACTIVE RWY SINCE WE AT ALL TIMES HAD BLUE TAXI LIGHTS IN SIGHT. THAT WHOLE RAMP AREA N OF RWY 27/9 IS VERY POORLY MARKED FOR ACFT TAXIING OUT OF WHERE WE CAME FROM.