PA28 PLT AND RADIO TECHNICIAN ON A GPS CERTIFICATION FLT EXPERIENCE NMAC WITH ANOTHER ACFT.

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I WAS FLYING A PA28-181 OUT OF SEE TO CERTIFY A GARMIN 430W WHICH HAD JUST BEEN INSTALLED BY THE LCL RADIO SHOP. MY ROLE AS PIC WAS TO FLY THE ACFT WHILE THE TECHNICIAN FROM THE RADIO SHOP VERIFIED THE OP OF THE NEWLY INSTALLED GPS EQUIP. WE DEPARTED FROM GILLESPIE VFR TO RAMONA RNM AND THERE COORD WITH THE TWR TO FLY OVERHEAD AND THEN MAKE A LOW PASS DOWN THE CTRLINE OF THE RWY 27. WE DEPARTED RAMONA AND CONTACTED SOCAL APCH TO REQUEST AN RNAV GPS 24 INTO CARLSBAD PALOMAR CRQ WITH THE HOLD AS PUBLISHED IN VFR CONDITIONS. SOCAL APCH TOLD US WE WERE CLRED TO FLY THE FULL APCH IN VFR CONDITIONS; NO SEPARATION SVCS PROVIDED. THE ATIS AT CRQ WAS RPTING 4 SM VISIBILITY HAZE OVCST 017. THE APCH WAS CLR OF CLOUDS WITH THE MARINE LAYER SITTING OVER THE ARPT. WE FLEW THE APCH TO VERIFY THE GS THEN BROKE OFF JUST INSIDE JABAL AND DEPARTED E BACK TOWARDS RAMONA. SOCAL CANCELED US AND WE SQUAWKED 1200. UNABLE TO FLY THE HOLD VFR AT CRQ; THE RADIO SHOP TECHNICIAN THEN SAID WE WOULD FLY THE RNAV GPS RWY 9 INTO RAMONA AND HOLD AS PUBLISHED AT ROBNN. WE CONTACTED THE TWR AT RAMONA AND REQUESTED THIS; TELLING THEM WE COULD MAINTAIN ALT WITHOUT THE NEED TO STEP DOWN. WE WERE REQUESTED TO FLY VFR AT 3600 FT MSL TO REMAIN ABOVE THE 2400 FT MSL TFC IN THE PATTERN. AT THE MISSED APCH POINT WE TURN TOWARDS ROBNN TO THE HOLDING FIX. AS WE APCHED ROBNN THE RADIO SHOP TECHNICIAN SHOUTED TFC AND I IMMEDIATELY SAW WHAT LOOKED LIKE ANOTHER PA28-181 PASS UNDERNEATH US APPROX 200-300 FT BELOW FROM A SLIGHT QUARTERING ANGLE R TO L. THIS WAS THE FIRST TIME I HAD EVER FLOWN A GPS CERTIFICATION FLT. THE TECHNICIAN KEPT REFERRING TO HIS CLIPBOARD BY HOLDING IT IN FRONT OF HIS FACE WHICH BLOCKED MY ABILITY TO SEE OUT OF THE R SIDE OF THE WINDSHIELD WITHOUT CONSTANTLY LEANING FORWARD. THE VISIBILITY WAS ALSO MUCH REDUCED DUE TO THE HAZE AND EVEN PREVIOUSLY CALLED TFC BY ATC WAS VERY DIFFICULT TO SPOT. I WAS AWARE OF THE PLAN TO FLY THE APCH INTO CRQ AND HOLD; BUT WHEN THAT WASN'T POSSIBLE DUE TO THE CLOUD LAYER; I WASN'T AWARE WHAT THE BACKUP PLAN WAS. IF I WERE TO FLY THIS EVER AGAIN; I WOULD DO A THOROUGH PREFLT BRIEFING WITH THE TECHNICIAN PRIOR TO THE FLT COVERING ALL CONTINGENCY PLANS. I WOULD ALSO REQUEST FOR CONTINUAL FLT FOLLOWING WITH SOCAL APCH AND REQUEST AN ALT THAT WOULD ALLOW THE ABILITY TO TEST THE EQUIP AND PUT US ABOVE THE MAJORITY OF THE TFC IN AND OUT OF THE LCL ARPTS.