The run-down:
2004 E55 - was experiencing the infamous alarm siren coming on randomly and draining my batteries, of course, didn't realize it was the back-up siren going bad until I searched.

Removed the back-up siren, and ordered a new one.


What happened:
Replaced the smaller front battery (dead, was seeing the red battery message for the past month), removed the rear battery to charge it up as it was at 2 volts. Put the charged battery back in, and some how mis-read the +/- signs at the ends of the battery and crossed polarity for a split second (sparked when hooking up positive cable) . OK - so I flipped the battery around to the correct position and hooked up the cables. Car started right up, but now has left tail light, rear left fog light, and rear back up light error messages.

Removed the rear left tail lamp assembly to check the bulbs, and all the bulbs are in good working order (verified by swapping them around). My tail lamps seems a bit different (07-09 style); they have the individual boards on each side, but the bulbs can be removed from their sockets... they're probably aftermarket.


Symptoms:
All lights on the rear right side work, but the left rear only has the turn signal/emergency light, brake light (top LED portion), and the lower right side of the tail light working - the lower left side and back up lights do not come on. The left license plate bulb is not turning on either.


Diagnosis/What I did:
I probed the lower left light socket with lights turned on, and there is a constant 3.2v running through the positive wire on there, while the other working light sockets show 0v before a bulb is inserted. When the brake pedal is pressed, the brake lights come on as they should, but the right side back up light turns on for a good second before it turns off.

I cannot tell, or find, a fuse for the tail lights, so I pulled out every single fuse I can find in the engine compartment, front driver side dash, driver side trunk, and the ones next to the trunk battery one by one and checking them for burns/opens. Every single one of them are in good shape.


I need help, LOL:
Tried searching how to diagnose this further, and can only come up with a guess that one of the SAMs may be bad, but I don't know which one controls the tail lights...

Any suggestions, tips, guides, help are all appreciated while I wait for the new back-up siren to come in.

TIA!

 

this will give you an idea where to start (at least on the running/parking light):

https://mbworld.org/forums/e-class-w...ml#post7074506

 

Thank you. All wiring passed checks. Turned out to be the rear SAM. Replaced with a used one from eBay - lights all working fine now