I've searched all threads and found much info but am not sure how to start my diagnosis.
2003 C240 does not get driven much.
First symptom was front left driving light was on one night and would not go off. Took out key, put light switch to off, light stayed on.
Secondly, put key in ignition and got NOTHING. No click, no dash lights, no sounds, nothing. Could not shift out of Park.
Tried many times, then finally put in key, turned, and car would turn on (not start, just turn on). Had dash lights and could shift out of Park, but car wouldn't even try to start. No clicking, nothing.
Tried to charge battery, it shows good charge. 3 days of front light on had drained down battery so I charged it up. I cleaned the terminals. No change.
So its 50-50. Sometimes I put in key and get nothing. Sometimes I can see dash lights and shift to neutral so I can roll the car, but it won't even try to start.

I've read about bad keys, cam sensors, batteries, fuses, etc. But I don't know how to start a good logical diagnosis and fix.

Help!

 

To add a little more.... I looked at every fuse I could find, and all look clean and ok. Fuse 52 took out and is still good.
Importantly, this all started after wife took car, drove 100yds and smelled smoke so shut off. Was clearly something bound up and burning the belt. I tried to start, heard a terrible grinding noise for 2 seconds and then went away. Drove the 100 yards back home ok. I was worried about broken water pump, or alternator. Shut car off, then started ok. I know somethings wrong, but not sure how that is related to the "brick" status of the car.
I'd heard that the constantly on running light was a signal of trouble.
Have tried to pull OBD codes but reader constantly shows "Fail".

I'd take to dealer for a full diagnostics, but can't even get to the point of getting it there.