EMB120 CREW BECAME UNSURE OF THEIR POS MAKING AN APCH TO SBP IN IMC CONDITIONS.

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ON DSCNT INTO SBP; CTR ADVISED 'EXPECT VISUAL APCH RWY 29.' ATIS WAS CALLING A SCATTERED LAYER AT 5500 FT. HOWEVER; WE SEEMED TO HAVE A SOLID UNDERCAST COMING IN FROM THE N. CTR CALLED THE ARPT AT 12 O'CLOCK POS AND 10 OR 12 MI; I THINK. WE RESPONDED WITH REQUEST ILS RWY 11 WE HAVE A SOLID UNDERCAST. CTR REPLIED 'ROGER; TRACK LOC INBOUND DSND TO 4000 FT.' WE BEGAN DSCNT AND TUNED IN LOC (NO DME); HE THEN CLRED US ON THE 'ILS RWY 11 CIRCLE TO LAND' APCH AND TOLD US TO CONTACT TWR. AS WE WERE IN THE MIDDLE OF OUR DSCNT CHKLIST (CALLING COMPANY) ETC; WE DID NOT IMMEDIATELY TALK TO SBP TWR. I NOTICED THE CAPT DIALED IN PRB VOR TO GET A FIX ON OUR POS RELATIVE TO CREPE. WE WERE PAST IT AND CONTINUING DSCNT. AS WE CONTACTED TWR WE BROKE OUT OF THE CLOUDS; BUT COULD NOT FIND THE ARPT AT OUR 12 O'CLOCK POS OR ANYWHERE. TWR TOLD US TO RPT DOBRA; WE SAID 'ROGER.' WE STILL COULD NOT FIND THE ARPT AND DID NOT TRULY KNOW OUR POS. WE FINALLY DETERMINED OUR POS VISUALLY UPON SEEING PISMO BEACH. WE TOLD TWR WE BELIEVED WE WERE S OF THE ARPT AND WANTED TO TURN BACK (NO RADAR AT SBP TWR). TWR RESPONDED THAT WE WERE NOT 'IN SIGHT' AND TO PROCEED TO THE ARPT UNDER VFR. TWR ALSO WANTED TO KNOW IF WE HAD CANCELED IFR WITH CTR (CLRLY HE WAS AS CONFUSED AS WE WERE). WE TOLD TWR 'NO; AND WE WERE TURNING BACK TO THE ARPT.' TWR THEN TOLD US THAT THERE WERE 2 ACFT IN OUR VICINITY BUT HE DID NOT KNOW THEIR POS WITH RESPECT TO OURS; AS WE WERE STILL NOT IN VISUAL CONTACT WITH HIM. AS WE ROLLED OUT OF OUR TURN ONTO (APPROX) A 320 DEG HDG TO THE ARPT TWR CLRED US FOR A VISUAL RWY 29 AS WELL AS TFC ON FINAL. ONCE WE CALLED TFC IN SIGHT; HE CLRED US TO LAND RWY 29. WE LANDED SBP UNEVENTFULLY WITH NOTHING FURTHER TO RPT. OBVIOUSLY; WE SHOULD HAVE NEVER ACCEPTED THE APCH CLRNC WITHOUT POSITIONAL AWARENESS. BUT WE ALSO FEEL CTR SHOULD NOT HAVE ISSUED A CLRNC WITHOUT SOME POSITIONAL RPT (IE; '6 MI FROM DOBRA CLRED ILS RWY 11 APCH'). HAD CTR TOLD US A POS OR GIVEN US A HEADS UP WE WERE INSIDE THE FAF (IE; '2000 FT ABOVE GS INTERCEPT; DO YOU REQUIRE A TURN?') IT WOULD HAVE BETTER PREPARED US FOR WHAT TO EXPECT UPON COMING IN VISUAL CONTACT WITH THE GND. THE POTENTIAL FOR INCIDENT/ACCIDENT WAS VERY HIGH IN MY OPINION.