Good point, there are 2 switches on the motherboard that have to be correct.
Switch 1 (Closest to edge of case) only transitions through the out position (ie In – Out – In) each time the ESL is locked or unlocked. Switch 2 changes state completely each time the ESL is locked or unlocked.

 

I used this stuff to coat inside and all over the outside if everything in the rear when I did my own subframe. I read the factory ones were not coated very well inside because they were manufactured in a nice climate country and they don't coat inside much because off expensive and they didn't see the need.

Dunno how well it's gonna last but it's better than nothing inside.

https://amzn.to/3OO0YC0

 

I'm on tha MA coast and park my car 10 feet from the ocean so I deal with major rust and corrosion. I see rust forming in rock chips on the hood never mind exposed metal parts underneath the car. Luckily my rear subframe looks ok and not one of the problem subframes. Not sure if it has to do with where the part was specifically manufactured or put together.

Parts I have replaced due to severe rust are:

- Front struts. Spring broke while parked.
- Rear shocks and springs. Lower isolators worn down to nothing.
- Rear brake dust shields. Crumbled apart causing parking brake pads to come loose.
- Transmission pan. Though extremely rusted it was still surprisingly solid.
- Exhaust clamps. Bolts completely rusted to nothing I was able to pull clamps off by hand.
- Underbody heat shield fasteners. Heat shielding was fallen down. Replaced with the plastic 10mm nuts used to hold the wheel well liners and holding up fine.
- Passenger rear brake line and hose. Looks like the drivers side had been replaced at some point.
- Passenger side exhaust manifold was severely corroded and replaced. The metal was pitted causing exhaust leak.

Both rear spring control arms look pretty rusted on the surface and I have two great condition used replacements to install this summer. Several rear control arm bushings look beat so will be replacing them. One of the exhaust flanges is cracked and broken so I bought a good condition used section of exhaust to replace it. Still have a middle section of brake line that looks rusted so I will be splicing out that part.

I started spraying corrosion inhibitor under the car 2yrs ago. Wish I started sooner when I first got the car. I tried products I can pick up locally like Fluid Film, Blaster Shield and CorrosionX. The CorrosionX seems to hold up the longest and its what I use from now on. Its more expensive by at least double but I only go thru 2-3 cans a year. They have three kinds, regular CorrosionX which is very thin, CorrosionX XD which is a little thicker then CorrosionX HD which is thick stuff. Sprayed the HD on my new transmission pan 2yrs ago and its still on and no need to respray. But the HD is only 12oc cans and the most expensive of the three. Worth it if you want to spray and forget about it. I like the XD and its what I have on hand. Safe for electronics also. This winter I had a snow blower electric starter that wouldnt work, just made a humming noise and wouldnt spin. I took it apart and sprayed CorrosionX XD all inside around the motor and after awhile was able to pull the motor out of the case. Cleaned up the rust and it works fine now. I think the Fluid Film and Blaster Shield are good products too but after the winter looked all washed off.

 

@TimC300 Well Tim, the saga is finally over. I had the car towed to a lock smith after MB dealership fried my ESL. Had the emulator installed and so far the car is driving mint. Now the car has the extended warranty new subframe, a fresh alignment,new battery and airbags recall. I prob should have got a second key made but I cheeped out. Now a fresh oil change and maybe the brake lines like you suggested. Honestly, you should at the very least pull the ESL from your steering column while it still works and unlocked and zip tie it under the kick plate so if it does fail, you wont get lock up like me.