AN LR60 FLC FAILS TO FOLLOW THE DEP PROC OUT OF MMSD WHEN RUSHED FOR TKOF.

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ON MAR/XA/99 AT SAN JOSE DEL CABO; MEXICO; WE CALLED FOR A TAXI CLRNC AND ADVISED ATC THAT WE WERE READY TO COPY OUR ATC IFR CLRNC. ATC TOLD US TO TAXI TO RWY 34 VIA TXWY A AND HOLD SHORT ON TXWY A; SO WE DID. NEXT THING WE WERE INSTRUCTED BY ATC TO BACK TAXI DOWN RWY 34 AND DO A 180 DEG TURN AND ADVISE WHEN READY FOR DEP. AS WE WERE BACK TAXIING FOR APPROX 5000 FT OF RWY; ATC THEN READ AN IFR CLRNC 'CLR TO KSDM; VIA AMARI 1 BRAVE DEP (SID) FLT PLAN RTE; FL390; ETC. BEFORE THE CREW COULD VERIFY THE SID DEP PROCS; WE WERE CLRED FOR AN IMMEDIATE TKOF; B727 ON SHORT FINAL. THE CAPT ADVISED THE FO TO FLY RWY HDG WHILE HE WAS PROGRAMMING THE FMS FOR THE SID. AT THAT TIME ATC ADVISED THAT WE WERE OFF COURSE AND SHOULD BE TURNING N TO INTERCEPT THE 340 DEG RADIAL. THE CAPT THEN REQUESTED DIRECT TO LAP THEN AS FILED. WE WERE CLRED TO LAP FLT PLAN RTE. CORRECTIVE ACTIONS: ATC SHOULD GIVE THE ATC IFR CLRNC ON THE TXWY OR BEFORE; NOT WHILE TAXIING DOWN THE RWY. THIS WILL GIVE THE FLC A CHANCE TO RESPOND AND PROGRAM THE FMS'S FOR DEP.