ACR BEGINNING DSCNT HAS NMAC WITH LIGHT TWIN.

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WE JUST PASSED CARDI INTXN ON THE CARDI 2 ARR INTO SAN DIEGO AND WERE CLRED FROM 15000 FT TO 7000 FT DIRECT THE MZB VOR. I HAD JUST FINISHED WRITING DOWN THE SAN ATIS AND COMPUTING SOME LNDG PERFORMANCE DATA WHEN I DECIDED TO LOOK FOR TFC. THE SECOND I LOOKED OUT; I SAW A LIGHT TWIN ENG DIRECTLY IN FRONT OF US; MOVING L TO R (HDG APPROX 220 DEGS). THE FO SAW THE PLANE AT THE SAME TIME I DID AND IMMEDIATELY HE GRABBED THE YOKE AND ROLLED THE PLANE INTO A 20 DEG L BANK; DISENGAGING THE AUTOPLT. THE LIGHT TWIN PASSED BY US LESS THAN 100 FT HORIZLY AND LESS THAN 10 FT VERTICALLY; AT OUR 1 O'CLOCK POS. WE WERE HDG 140 DEGS; DSNDING AT 2000 FPM AND 260 KIAS. WE IMMEDIATELY NOTIFIED THE APCH CTLR; WHO DID NOT HAVE THE ACFT ON RADAR. UPON LNDG AT SAN THE CAPT CONTACTED THE APCH CTL SUPVR AND FILED A RPT. AT THE TIME OF THE INCIDENT A RARE ECLIPSE OF THE SUN WAS OCCURRING; AND WE WERE TOLD AFTERWARD THERE WERE MANY; MANY TARGETS ON THE RADAR. WE THEORIZE THAT THE LIGHT TWIN WAS A SIGHTSEER (OF THE ECLIPSE) AND IN ALL PROBABILITY NEVER SAW US. ALSO; WE WERE NOT TCASII EQUIPPED; SO EVEN IF THE PLANE HAD AN OPERATING TRANSPONDER; WE WOULDN'T HAVE BEEN ABLE TO DETECT HIS PRESENCE SOONER BY SEEING HIM ON THE TCASII DISPLAY.