RPTR'S TAXI PATH WAS VIOLATED BY ANOTHER ACFT WITHOUT CLRNC AND ON THE WRONG FREQ.

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I LANDED THE ACFT ON RWY 18R AND THE CAPT CLRED THE RWY AT TXWY 29. TWR CLRED US ACROSS RWY 18L AND TOLD US TO CONTACT GND CTL ON 121.8. GND CTL TOLD US TO 'TURN R ON THE OUTER TXWY (FOXTROT) AND THEN L ON 31 AND CONTACT GND CTL ON THE E SIDE OF THE ARPT ON TOP OF THE 31 BRIDGE.' AS WE TAXIED S ON FOXTROT; THE CAPT BEGAN TO SLOW THE ACFT AS HE WAS LOOKING L IN PREPARATION FOR THE L TURN FROM FOXTROT ONTO 31. JUST PRIOR TO TURNING; I OBSERVED ANOTHER MD-88 CLRING RWY 18L ON TXWY 31. THE OTHER ACFT WAS NOT SLOWING DOWN AND HAD NOT CONTACTED GND ON 121.8. I TOLD THE CAPT THAT THE OTHER ACFT WAS COMING FAST AND DID NOT APPEAR TO BE SLOWING TO A STOP. THE CAPT IMMEDIATELY STOPPED THE ACFT PRIOR TO TXWY 31. THE OTHER ACFT FINALLY STOPPED (AFTER VEERING TO THE R) AND WELL INTO THE INTXN AT FOXTROT AND 31. GND CTL CALLED US AFTER WE HAD STOPPED AND STATED HE WAS NOT IN CONTACT WITH THE OTHER ACFT. THE OTHER ACFT PLT IMMEDIATELY STATED HE HAD PRIORITY SINCE HE WAS XING RWY 18L. GND CTL TOLD HIM HE DID NOT HAVE PRIORITY AND THAT HE HAD NOT CHKED IN AFTER XING THE RWY. GND CTL TOLD THE OTHER ACFT TO CONTINUE TAXIING; AND FOR US TO FOLLOW HIM ON TXWY 31. POSSIBLE CAUSES: 1) THE OTHER ACFT MISTAKENLY THOUGHT HE HAD PRIORITY. 2) LATE AT NIGHT DFW TWR WILL OFTEN CLR AN ACFT ACROSS RWY 18L AND HAVE THE ACFT CONTACT GND CTL E ON TOP OF THE 31 BRIDGE. THIS NORMALLY IS DONE WHEN THE TWR CTLR IS CTLING LNDG AND TAXIING ACFT ON HIS SIDE OF THE ARPT. 3) THE OTHER ACFT WAS ON THE WRONG RADIO FREQ. 4) THE OTHER ACFT DID NOT SEE US. IF WE HAD NOT STOPPED OUR ACFT PRIOR TXWY 31; THE OTHER ACFT WOULD LIKE HAVE HIT US. AS IT WAS; NEITHER ACFT WAS CLOSER THAN 100 FT.