BE99 SINGLE PLT INCURS BOTH NAV AND HDG PROBS WHILE IN IMC.

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WHILE FLYING A BE99 FOR A PART 135 CARGO COMPANY; IT APPEARED AS MY 2 VOR RECEIVERS WERE NOT GIVING CORRECT INFO; AS WELL AS HAVING AN INACCURATE ADF RECEIVER AND A GYRO WHICH WAS 20 DEGS OFF MY COMPASS HDG WHILE INITIATING AN ILS APCH IN IMC. AFTER DOING SOME TROUBLESHOOTING I WAS ABLE TO ASCERTAIN THAT ONE OF THE VOR RECEIVERS WAS STILL IN GPS MODE AS I HAD A GPS (VFR) ONLY ON BOARD. NOW I HAD 1 OPERATIVE VOR RECEIVER. ALSO MY HDG INDICATOR WAS 20 DEGS OFF WHEN I SET IT TO THE WHISKEY COMPASS. I LEARNED THAT IT WAS PROHIBITED TO SET THE HDG INDICATOR WITH THE USE OF WINDSHIELD HEAT ON. THIS INFO WAS NOT PLACARDED IN THE AIRPLANE. BECAUSE OF THE INACCURATE HDG INDICATOR; THE ADF NEEDLE WAS ALSO 20 DEGS OFF. AFTER THESE PROBS WERE RESOLVED; I WAS ABLE TO COMPLETE THE APCH SUCCESSFULLY WITH THE USE OF 1 VOR RECEIVER. MY CONCLUSION IS THAT FOR AN ACCIDENT TO OCCUR SEVERAL BREAKS IN A CHAIN MUST OCCUR. I WAS SINGLE PLT IMC AT THE TIME WITH LESS THAN 100 HRS PIC IN THE ACFT. FORTUNATELY I WAS ABLE TO CORRECT THE SITUATION. ONE OF MY RECOMMENDATIONS IS TO PLACARD 'DO NOT USE WINDSHIELD HEAT WHILE RESETTING THE HDG INDICATOR' ON THE ACFT.