RICHLAND FLT #101, A BANK CHECK COURIER FLT, DEPARTED VFR WITH 180 LBS OF CARGO. SIX MINUTES PRIOR TO THE CRASH, THE LAST RADIO TRANSMISSION FROM THE ACFT WAS HEARD CONCERNING A ROUTINE LANDING WX ADVISORY. THE ACFT CRASHED APRX 45 DEGS NOSE LOW, GEAR & FLAPS UP IN FLAT, OPEN TERRAIN. A 5 DAY HISTORY ON THE PLT REVEALED THAT HER FOOD INTAKE WAS ERRATIC AND THAT SHE WAS ON A HECTIC SCHEDULE. HER FOOD INTAKE AND AMT OF REST IN THE 24 HRS PRECEEDING THE ACCIDENT IS UNKNOWN. A RELATIVE STATED THAT SHE WAS PSYCHOLOGICALLY WELL ADJUSTED. CONCERNING THE WRECKAGE, CONTROL CONTINUITY COULD NOT BE CONFIRMED DUE TO FIRE DAMAGE. AN EXAMINATION OF THE ENGINE DID NOT REVEAL ANY EVIDENCE OF A PRE-IMPACT MALFUNCTION. THE FOLLOWING AUTOPILOT MODE LIGHT BULBS HAD STRECHED FILAMENTS: ALT (UPPER), HDG (UPPER), FD (UPPER), VOR, NAV (UPPER), AP (UPPER) AND BC (UPPER). LASTLY, AN AUTOPSY REVEALED THAT THE RADIO MICROPHONE WAS IN THE PLT'S RIGHT HAND AT IMPACT.