As the acft rotated for tkof, the cargo door (CD) opened. The crew contd tkof & turned to a dwnwnd leg to lnd on rwy 17. They answered twr instrns to rprt 'base.' This was the last tmtn they rcvd, tho they tried svrl times to ctc twr or apch ctl. As the acft turned on base leg (with a tail wnd at tfc pattern alt), radar data indcd acft would cross the rwycenterline. Radar ctc was then lost; the acft crashed in a steep dscnt. There was evidence that bfr flt, the 1st officer misinterpreted the external lockpin manual ctl handle psn to mean the CD was locked (as a result of incorrect markings). Also, there was a malfunction of a CD open warning lgt sw, preventing opn of the warning lgt. Additionally, svrl svc bulletins (SB's) concerning the CD had not been complied with. These SB's would have provided an extra safety feature of the CD warning lgt sys, a door venting sys & a lockpin viewing window for the door. There was no emerg proc or specific guidance to aid DC-9 crews for this situation, though svrl CD openings had occurred. (See: NTSB/AAR-90/02)