THE ACFT WAS ON A NIGHT IFR FLIGHT AT 7000 FT MSL IN IMC WHEN THE PLT REPORTED THE DIRECTIONAL GYRO WAS INOPERATIVE. THE ACFT WAS OUTBOUND FROM THE PULASKI VOR ON V45 HEADED SOUTHEAST. THE PLT FURTHER REPORTED THE TURN&BANK AND THE MAG COMPASS WERE OPERATIVE. SHORTLY AFTERWARDS THE PLT WAS UNABLE TO MAINTAIN THE CORRECT COURSE. THE ACFT EVENTUALLY REVERSED COURSE. WHILE ATC WAS TRYING TO HELP THE PLT RETURN TO THE CORRECT COURSE THE ACFT DESCENDED AND COLLIDED WITH THE GROUND. WITNESSES NEAR THE ACCIDENT SITE REPORTED POOR VISIBILITY, LOW CLOUDS, AND FOG. THE AIRCRAFT'S IMC CAPABILITY COULD NOT BE ACCURATELY DETERMINED DUE TO THE EXTENSIVE FIRE AND IMPACT DAMAGE. EXAMINATION OF THE ENGINE DRIVEN VACUUM PUMP DID NOT REVEAL CONCLUSIVE EVIDENCE OF FAILURE. THE ACFT WAS EQUIPPED WITH A STANDBY VACUUM SYSTEM, HOWEVER, OPERATION OF THE SYSTEM COULD NOT BE CONFIRMED. THE PLT HAD FLOWN THE FIRST LEG (4.0 HRS.) TO DROP OFF PAX AND WAS RETURNING TO HIS HOME BASE. THE ACFT HAD FLOWN ABOUT 3 HRS. ON THIS LEG.