So, it looks like I'm hunting down a tall relay with metal housing with two 10 amp fuses in it. BUT I CAN'T FIND IT.
It's not in the Main fuse box, not by the battery under the seat.
Doesn't appear to be in the box by the passenger firewall in the engine... BUT
Could it be inside this K40 relay I see in this page?
Mercedes-Benz SLK 230 K40 Overload Protection Relay Repair | 1998-2004 | Pelican Parts DIY Maintenance Article

Bueler? I don't want to crack this open if I don't have to.

I have weird random codes that happen when I plug in a door light.

Don't bother opening the W210 K40 box. It isn't setup like that R170 box. All you'll see is the back side of a PCB with everything attached hidden in the half of the case containing the connectors. I replaced the K40 when the wagon wouldn't communicate with SDS. All the fuses and the external relay were fine. In fact I transferred all the loose bits to the replacement K40. It's chatty with SDS now.

Sixto
98 E320s sedan and wagon

This is not necessarily true. I reflowed all the solder joints in my K40 to fix a stalling problem.
http://youtu.be/eB0iCiIeH9k
There are several different versions of the k40 on the w210 depending on the engine and model year.

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CENSORED due to not family friendly words

I opened it up.
The joints looked good.
I tried to clean up a couple of them, but my soldering iron was not up to the task really.
It has seen better days since I used it to fix my plastic canoe.. Which came out great.

Ah so. Mine both look like this -

Sixto
98 E320s sedan and wagon

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