HDG DEV HDG EXCURSION -- CAPT COPIED THE WRONG HDG; BUT FO AND THE NAV SYS BRING HIM BACK TO THE CORRECT COURSE.

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AFTER TKOF FROM DTW (RWY 3C) ON THE ST. CLAIR ONE (ASSIGNED 035 DEG HDG) GOING THROUGH 6500 FT DEP GAVE US A TURN TO 095 DEGS DIRECT TYCOB. I STARTED A R TURN TO A HDG OF WHAT I THOUGHT WAS ASSIGNED HDG OF 195 DEGS. AT THE SAME TIME; I TOLD THE COPLT TO ACTIVATE THE GNS. GNS WAS NOT WORKING PROPERLY; SO I TOLD THE COPLT TO REPROGRAM. GOING THROUGH A 140 DEG HDG; COPLT LOOKED UP AT HDG AND SAID 'WE WERE ASSIGNED 095 DEGS.' I SAID 'I THOUGHT IT WAS 195 DEGS.' ABOUT SAME TIME GNS COMES ON- LINE INDICATING A TURN BACK TO THE L TO GO DIRECT TYCOB. I IMMEDIATELY STARTED A TURN BACK TO GO DIRECT TYCOB. DUE TO CTLR BEING BUSY; COPLT DIDN'T READ BACK ASSIGNED HDG OF 095 DEGS; WHICH IF HE HAD; PROBABLY WOULD HAVE PREVENTED THE HDG DEV. IN THE FUTURE; THE COPLT WILL READ BACK ALL HDGS AND ALTS ASSIGNED; REGARDLESS OF HOW BUSY THE CTLR APPEARS TO BE. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING: THE RPTR WAS FLYING A FALCON 20 EQUIPPED WITH A GLOBAL NAV SYS. HE ADMITS THAT THE GNS ACTIVATION PROB DISTRACTED HIM A BIT AND THAT HE WAS IN ERROR ABOUT THE HDG. HE HEARD; 195 DEGS NOT 095 DEGS; THE CTLR'S INSTRUCTIONS DURING A DISCUSSION ABOUT THE GNS. THE FO WAS NOT CONFUSED. THE CAPT SAID THAT THE FLC FOLLOWED GOOD COCKPIT RESOURCE MGMNT THROUGHOUT THEIR FLT.