Installed battery in backwards

Can you tell us what the issues are with the vehicle? Were these not there prior to hooking it up backwards? If not then fixing it is not extremely hard but it may be costly. Normally when this happens it damages the circuit boards of the modules as it sends power the wrong way. However, the wiring and most sensors don't care which way the power goes. They clearly only operate if the power flow is wrong but they are not damaged.

Let's start with what codes that you have and what issues you are seeing and we can go to the next step.

https://www.2carpros.com/articles/checking-a-service-engine-soon-or-check-engine-light-on-or-flashing

I had the car towed to a local garage and they told me the electrical is "fried" but this is not their area of expertise. They said they smelled that something burned up. The key wont turn in the ignition and when jumped, some of the electrical components work but many do not. We had the car towed back to my house. I recently put a couple thousand into it and I am willing to invest more since I have so much in it already. Would Mercedes be better prepared to troubleshoot and repair or is it indeed "fried" and cut my losses. thank you!

If they can smell the electronics then something is "fried" but it may just be a module and it is unlikely to be wiring. If you like the car then I would spend the money on getting someone with experience on this car to look at it. I would just call around to some other shops and see if they work on them. You don't have to take it to the dealer but if nothing else, it may be worth paying for them just to tell you what it will cost then you know for sure if it is worth it to you to put the money into it. That is just what I would do. However, it you are fed up with it then that is fine as well.

Thank you for your advice. I am willing to put a couple grand into it as it is far less expensive than finding and buying a replacement vehicle. I will ask around to see if there is a shop that is better able to troubleshoot electrical issues caused by human stupidity. Thanks!

You are welcome. Let us know how this turns out. Thanks for the update.