My w215 has been really acting strange. What first happened is when I turned the car off one day the dashboard stayed illuminated..along with the brake lights and the esp abs lights on. So I had to disconnect the battery each night to stop the power drain. Then one day I was driving home and the car started misfiring on several cylinders. I turned the car off...and when I turned it on...(in park) the throttle accelerated to 5k rpms. And stayed there until I held the brake and put it in drive...when I get home I turn the car off...and disconnect the battery..when I go to start it in the morning ...it sounded strange ...like a strange sound during cold start up.. so I turn the car off...and try to start it again...but the key won’t turn at all! The dash lights come on when I put the key in..but it won’t turn...so I disconnect the battery and try to start it...and it starts right up. I was charged 150 by a mechanic to run the codes...but he didn’t tell me anything...or what I should do to fix the problem. I will upload pictures of the fault codes. Please someone help me figure out where to start...

Your first issue, regarding the speedometer sounds like the very common speedometer failure issues in the W215/220 platform... I’d heard the speedometer issues were resolved in the facelift models but who knows if you have an 2002 cluster in your 2003. This was a recall so look online to see if your car is due for any recalls first

Misfiring and random rpm spikes could possibly be a failing crankshaft position sensor, you should have one of these handy anyways with all the heat the exhaust generates and reputation for failure. Sometimes a failing/dirty MAF sensor can cause similar problems, although I’ve never heard of an MAF to cause an rpm spike to 5k rpms...

ultimately i think your issue involves a bad ground connection. Besides the negative battery terminal, there are several points of ground on these cars for each of the electrical systems.

Lastly i I would recommend following up on any service maintenance intervals that may have been skipped.. these cars call for extensive service rituals, and in my experience quite a few issues people have with their used Benz gets resolved when the owner catches up with the servicing. I’m not talking about oil changes , as much as I’m talking about valve cover gaskets , spark plugs, ignition coils etc

Agree that’s most like a voltage issue , how old is your battery ?

get a fresh new battery and check all cables and grounds before you replace any electronic part