The p1717 code sets when the circuit in the transmission range switch is sensed to be open while the transmission is mechanically shifted into reverse, this can happen a couple ways, either a faulty range switch, improper adjustment of the range switch, a broken wire in the circuit, or a faulty pcm. From your description I would suspect the switch is out of adjustment since you can get it started by moving the shifter, so what's happening it the switch is between positions when you're in gear, so you may be able to turn the switch to better line up with the shifter positions and solve the Mercedes star C3. The range switch is located on the side of the transmission, you should be able to access it through the drivers side wheel well, it'll have 2 bolts holding it to the transmission, if you loosen them you can reorient the switch, what you'll want to do is try reorienting it then checking if the transmission position on the dash switches on the dash when you switch gears, if it seems to be off try again until it's switch position at the same time it's actually switching positions.