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The problem most likely is the cylinder lock cable that goes from Gear shift selector (P / N/ R / D / OD) to ignition switch. If you take off center console to allow access to shift selector there is a screw on the left hand side of selector main body that attaches to cable. The cable is self adjusting according to service manual but I had similar problem. I loosened screw and shifted the selector from P to N and back to P then retightened screw. Not sure if this would be directly linked to servicing the alternator but who knows what the mechanic did in their process of fixing the alternator. Good mechanic will at least do some steps to ensure that is indeed the problem with the vehicle and not waste your money or time. Im assuming the vehicle wouldn’t start if the alternator was bad and left the battery discharged. So any mechanic is going to test starter, ignition switch, ASD relay, Alternator, and so on. To test ignition switch he might have unplugged the cylinder lock cable from ignition switch. i hope this helps.