WRONG RWY TKOF BY A PA24 PVT PLT AT GRR; MI.

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DEPARTED RWY 17 INSTEAD OF RWY 35. I HAD A COMBINED NAV/COM/GPS INSTALLED AND IT WAS READY FOR PICKUP; SO THEY SAID. I HAD LANDED RWY 26L ON WAY IN FOR PICKUP. DEPARTED RWY 17 FOR FIRST TEST FLT WITH A LCL AVIONICS TECHNICIAN ON BOARD. PROBS: IMMEDIATELY RETURNED TO RWY 26R. RADIO TECHNICIAN PERSON WAS VERY GREEN AND WAS ASKING ME (GREENER STILL) HOW TO WORK RADIO FOR SECOND DEP. I HAD ASKED FOR PROGRESSIVE TAXI INSTRUCTIONS FROM GND BECAUSE OF WORKLOAD IN CHKING OP OF RADIO. CAME TO STOP BEHIND CESSNA WAITING ON TXWY A; HOLDING SHORT OF RWY 35/17; STILL PLAYING WITH RADIOS. HE TOOK OFF; I ASSUMED RWY 17; BUT HE CONTINUED 180 DEGS TO RWY 26L DEP. I WASN'T MONITORING TWR AT THAT TIME AND HADN'T NOTICED THE 180 DEGS. I THEN MOVED UP AND CONTACTED TWR; THEY CLRED ME FOR TKOF. I TURNED R ON RWY 17 INSTEAD OF RWY 35; TOOK OFF MAKING SURE THAT RWY 26L WAS CLR. I DISCOVERED ERROR WHEN TWR CALLED AND SAID MAKE IMMEDIATE R TO 350 DEG HDG. CONTRIBUTING FACTORS: NOT PAYING TOTAL ATTN TO ENVIRONMENT; WAS HELPING TECHNICIAN DO ALL HIS CHKS; I HADN'T MADE A LOT OF INTXN DEPS; SAW CESSNA TURN R EARLIER AND JUST DID SAME AS HIM. MAYBE IF TWR HAD SAID 'TURN L FOR RWY 35 DEP' INSTEAD OF JUST 'CLRED FOR TKOF' I WOULD NOT HAVE MADE THIS ERROR IN FOLLOWING CESSNA. I LEARNED THAT I SHOULD BE IN CTL OF AIRPLANE AND ENVIRONMENT SURROUNDING IT.