ACR MLG UNAUTH TAXI AND RWY TRANSGRESS CAUSES A GAR AT SAT.

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AFTER PUSHBACK FROM GATE AND UPON ISSUANCE OF TAXI INSTRUCTIONS TO PROCEED TO RWY 12R; THE CAPT BEGAN TAXIING. THE TAXI CHKLIST WAS CALLED FOR AND I EXTENDED THE SLATS/FLAPS. I AM ESPECIALLY THOROUGH WITH PROPER POSITIONING OF THE SLATS/FLAPS AND PLACE MUCH EMPHASIS WITH THE CORRECT COMPLETION OF THE TAXI CHKLIST. AS THE ACFT WAS TURNED TO THE R AWAY FROM THE JETWAY; I SAW CONSTRUCTION ON THE RAMP NOW TO THE CAPTS L AND THEN ANOTHER TAXIWAY THAT WAS ILLUMINATED IN RED LIGHTS 'NO ENTRY.' THIS I SUBSEQUENTLY LEARNED WAS THE HIGH SPD TURNOFF FOR RWY 12R. I CLRED OUR ACFT ON THE R TO AVOID OBSTRUCTIONS AND ALSO OBSERVED AN ACR Y MLG WHICH WAS INSTRUCTED TO FOLLOW US. AFTER CLRING; I CONTINUED WITH THE CHKLIST DOUBLECHKING SLAT/FLAP; CG AND STABILIZER SETTINGS. THE DIMLY Lit COCKPIT WITH POORLY LIT INDICATIONS AND MARKINGS DISTR ME FROM REALLY INSURING WHERE CAPT WAS TAKING US. AS MY ATTN WAS STILL FOCUSED ON THESE DETAILS HE STARTED A L TURN OFF THE RAMP ONTO THE PERPENDICULAR STUB BTWN THE RAMP AND WHAT I MISTAKENLY THOUGHT WAS ANOTHER TAXIWAY. AT THAT MOMENT HE MADE A STATEMENT ABOUT A FLY OR BUG FLYING ABOUT IN THE COCKPIT. HE THEN OPENED HSI DV WINDOW AND ANNOUNCED THAT WE WERE GOING TO REMOVE SAID INTRUDER. NOW MY ATTN WAS FURTHER ERODED WITH THIS PROBLEM ONLY TO FIND US IN ANOTHER L TURN; NOW ONTO RWY 12R. NOW THE UNISON 'OH; BLEEP'. THE SHARP CTLR WORKING LCL AND GND CTL SAW US AND THEN ISSUED A 'GAR' TO AN ACFT ON FINAL. JUST PRIOR TO ENTERING 12R I STARTED TO QUESTION BUT IT WAS TOO LATE. THE CAPT THEN TAXIED CLR OF 12R ON THE REVERSE HIGH SPD TAXIWAY AND PROCEEDED ON THE PROPER TAXIWAY TO 12R.