PA28; ON SHORT FINAL RWY 9L AT TMB; EXECUTES GAR DUE TO TFC ON RWY AS THE GND CTLR ADVISES THE LCL CTLR OF THE POTENTIAL CONFLICT.

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TMB IS A VFR TWR WITH A BRITE RADAR DISPLAY THAT FEEDS OFF OF MIA APCH RADAR. DUE TO A NEWLY ERECTED BUILDING; WE HAVE DEVELOPED A BLIND SPOT W OF THE FIELD ON THE BRITE DISPLAY. I WAS WORKING LCL CTL N. A P28B WAS ON FINAL RWY 9L; WITH A SMALL HELI IN TRAIL APCHING THE FIELD FROM THE W. I HAD NOT YET VISUALLY IDENTED THE P28B. I WAS RESOLVING POTENTIAL CONFLICTS TO THE E; AS WELL AS COORDINATING TURNING AN ACFT THROUGH THE LCL CTL S CTLR'S AIRSPACE WHEN C172 CALLED FOR DEP FROM RWY 9L. I LOOKED AT THE BRITE DISPLAY AND SAW A PRIMARY TARGET ON A 2 MI FINAL FOR RWY 9L AND ASSUMED IT WAS THE P28B. I KNEW THE HELI WAS FLYING AT OR BELOW 500 FT. DUE TO LINE-OF-SIGHT LIMITATIONS; HELIS AT LOW LEVEL OFTENTIMES DO NOT SHOW UP ON RADAR. I ATTRIBUTED THE LACK OF A SECOND TARGET TO THIS FACT. I TOLD THE C172 TO TAXI INTO POS AND HOLD (TFC WAS CLRING DOWNFIELD). FORTUNATELY; THE GND CTLR WAS ALERT AND POINTED OUT THAT THE P28B WAS ON 1/2 MI FINAL. JUST AS I KEYED MY MIKE TO SEND THE P28B AROUND; THE PLT INITIATED A GAR HIMSELF. ADEQUATE SEPARATION WAS MAINTAINED. IN HINDSIGHT; THE PRIMARY TARGET I OBSERVED N THE BRITE DISPLAY WAS THE HELI; AND THE P28B WAS NOT SHOWING UP AT ALL. THE BLIND SPOT HAS BEEN RPTED TO THE APPROPRIATE PERSONNEL; AND SOLUTIONS ARE BEING CONSIDERED.