RPTR WORKING 4 POS IN THE TWR INCLUDING GND CTL; CLRED THE WRONG VEHICLE TO CROSS A RWY WHEN A C172 WAS ON TKOF ROLL. 2 VEHICLES WERE ON GND CTL FREQ AND THE RPTR MISINTERPRETED WHICH VEHICLE CALLED AND AT WHAT LOCATION ON THE ARPT. LTSS OCCURRED.

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THE PROB AROSE WHILE I WAS WORKING GC/FD/CD AND CIC COMBINED. I HAD 2 VEHICLES ON FREQ WITH SIMILAR CALL SIGNS: MAINT X AND ADMIN X. ADMIN X WAS ON THE N SIDE OF RWY 8 AT THE APCH END OF TXWY L IN THE VICINITY OF TXWY F. MAINT X WAS AT THE APCH END OF RWY 13 IN THE VICINITY OF TXWY F. I HAD TALKED TO BOTH VEHICLES PRIOR TO THE EVENT AND WAS CONFUSED AS TO WHICH VEHICLE WAS USING WHICH CALL SIGN. WHEN MAINT X CALLED AND REQUESTED RELOCATION FROM TXWY F AND APCH END OF RWY 13 TO TXWY A AND TXWY L I IMMEDIATELY FOCUSED ON THE VEHICLE AT THE APCH END OF RWY 8 WITH THE THOUGHT THAT HE WANTED TO CROSS THE RWY AND CONTINUE HIS WORK ON THE SAME TXWY ON THE OTHER SIDE OF THE RWY. THE FACT THAT THE VEHICLE SAID HE WAS AT THE APCH END OF RWY 13 ESCAPED MY ATTN. WHEN I COORDINATED WITH LCL CTL TO CROSS RWY 8 AT THE APCH END OF RWY 8 BEHIND A TOUCH AND GO CESSNA I DID NOT EXPECT AND WAS VERY SURPRISED WHEN THE OTHER VEHICLE CROSSED FARTHER DOWN THE RWY APPROX 1800 FT IN FRONT OF THE DEPARTING ACFT. CONTRIBUTING FACTORS WERE THE SIMILAR CALL SIGNS; THE EXTRA WORKLOAD OF WORKING 4 COMBINED POS; AND MY CONFUSION ON THE IDENT OF THE VEHICLES. CORRECTIVE ACTION TO BE TAKEN WOULD BE TO CHANGE THE CALL SIGNS OF ONE OF THE VEHICLES TO BE LESS CONFUSING AND TO BE SURE OF WHICH VEHICLE IS TALKING BEFORE ISSUING XING CLRNC.