RWY EXCURSION AFTER ACFT EQUIP PROB MALFUNCTION AND DESTABILIZED APCH LNDG PROC ROLLOUT.

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UPON ENTERING THE TERMINAL AREA; THE CAPT AND I PREPARED THE MDT FOR A VISUAL TO 5R AT INDY. IT WAS MY LEG SO IT WAS TO BE MY LNDG. UPON TURNING BASE TO FINAL I CALLED FOR THE GEAR DOWN (IT WAS SELECTED) AND 1 OF THE 2 NOSE GEAR DOWN LIGHTS FAILED TO OPERATE. CAPT SAID TO DISREGARD SINCE THE CONDITION HAD HAPPENED PREVIOUSLY AND ALL OTHER FACTORS CONCURRED THE GEAR WAS DOWN. I THEN ASKED FOR FLAPS 16; 26 AND 40 DEGS AS PER THE SOP. WHEN THE FLAPS WERE DOWN; THE GEAR WARNING HORN CAME ON (IT'S TIED IN TO THE SAME SWITCH AS THE LIGHTS). IT WAS VERY HARD TO HEAR ANYTHING. CAPT THEN REACHED DOWN AND PULLED THE FLAPS FROM 40 BACK TO 16 DEGS; CAUSING A PITCH AND AIRSPD CHANGE. I WOULD HAVE RATHER HAVE NOT DEALT WITH BEING SO CLOSE TO LNDG. AT ANY RATE; I ADJUSTED FOR THE CHANGE AND INCREASED VREF AND VTH SPDS -- NO PROB. DURING THE LNDG THE HORN WAS STILL BLARING. DURING THE LNDG ROLLOUT I SELECTED GND FINE PITCH AS PER SOP AND AS WE PASSED THROUGH 60 KTS CAPT SAID -- YOU GOT TILLER STEERING (COMPANY PROC IS TO GIVE STEERING CTL TO CAPT AFTER 60 KTS) -- BUT HE WAS BUSY USING THE L-HAND PUSH- TO-TALK SWITCH (TALKING TO TWR; AND THUS COULD NOT USE TILLER TO STEER). I STARTED STEERING WHEN THE ACFT WAS AT 60 KTS AND THEN THE ACFT VEERED TO THE R. TRIED TO CORRECT BUT THE NOSEWHEEL STEERING WAS INEFFECTIVE (ACCORDING TO MANY CAPTS THE MDT NOSEWHEEL SYS SOMETIMES 'CUTS OUT' AND FAILS DURING THE LNDG SEQUENCE). CAPT SAW THE PLANE GOING TO THE R AND GRABBED FOR THE CTLS. HE THEN ATTEMPTED TO CORRECT THE CONDITIONS WITH NOSEWHEEL AND RUDDER/BRAKE CTL BUT THEY SEEMED INEFFECTIVE. HE HAD THE TILLER AND RUDDER AT FULL L BUT THE BIRD KEPT ON GOING FOR R. THE PLANE CAME TO A REST WITH THE R MAIN IN THE SOFT GRASS BUT NO DAMAGE TO THE PLANE OR RWY LIGHTS. WE CALLED FOR A TUG AND STARTED TO GO THROUGH THE SHUTDOWN CHKLIST.