Weird (to me at least) issue with our 2004 300m.
Has been sitting for at least 3 months. Tried to install new battery, that's when it got weird. Connected positive, no problem. As soon as I connect the negative, several systems come on at once (wipers, ABS motor, interior fan...) Inside none of those systems (except ABS I assume) are turned on, and the ignition switch is in the off position.
To boot, it doesn't respond at all to the ignition switch in any position (no dash lights, starter doesn't turn).
So I disconnected the negative, checked all the fuses and relays in the panel (all look good). Tried reconnecting and same issues. Pulled just the fuses for the systems that came on, and still no ignition switch response.
Anyone have an idea what's going on? If I can upload a video I'll try
Thanks (Adam in Colorado)

 

First be SURE you have the battery connected properly. B+ to PDC & Alternator and B- to Body Ground Post.
After that odds on bet mice/ critters have gotten into the wiring.
Insulation on some wiring of this era is like string candy to mice.
Have seen the area behind the fuse box chewed up on a few occasions.

 

Didn't see any initial signs of critters. Will do a closer look today though. I did try pulling only the fuses for systems coming on and still no dice on the dash lights or starter.
MIght also try pulling all of the nonessential fuses and seeing if that makes a difference.
Thanks for the ideas.

 

Battery is definitely hooked up correctly, but will check closer to see if there's a short on any of those wires. 🙂

 

Bruised ego here. Realized I had switched the terminals after all. That did it. Thankfully only looks like a couple blown fuses and no other damage.
thanks for the help!

 

Easy mistake to make thanks to the way the battery is buried in the wheel well. Great to hear it’s alive.
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