Q: i plugged my negative cable on my mercedes 2008 c300 before positive what could i have messed up

As long as you did not reverse the polarity of the battery (i.e., connect the negative terminal of the battery to the positive side of the vehicle electrical system and connect the positive terminal of the battery to the vehicle ground) or short circuit the battery what you are describing is harmless. Connecting the ground terminal last is simply recommended to avoid short circuits. If you didn’t short circuit anything, then you needn’t worry. If after replacing the battery you are experiencing electrical problems, that may be due to the PCM/ECM having lost stored information or possibly the engine immobilizer system is operational. If you are experiencing an electrical problem with the vehicle, please request an electrical diagnostic and a certified Mechanic, dispatched by YourMechanic right to your location, will get this resolved for you. If you have further questions or concerns, do not hesitate to re-contact YourMechanic as we are always here to help you.