I have reset the 4matic code(pin 5,3 flashes,brake light switch)on my 1992 300te several times over the years but I can't find a solution to this problem in the threads.
My home made code reader led stays red always.Whether I short the ground,or disconnect it completely led is on.No difference if key is on or off.

Removed and checked brake light switch with ohmmeter,working properly.

led goes out when module disconnected.

4matic dash light is illuminated.

I tried other pins and reader works fine.

When I disconnect the battery,led stays lit for 2 seconds before going out.

Any ideas.

The brake lamp switch has 2 functions & they are critical.

I would step up to the pump & spend the $15 for a new switch.

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Yes I agree,but I checked the switch with an ohmmeter and both circuits operate properly.The test LED stays on whether the switch is connected or not.
There should not be power in pin 5 but I have 12 volt reading.
I am presently backtracking the wiring(with diagram) and trying to find where/what the power source is.
There is obviously a short.
Car was parked for summer and I have had previous problems with mice getting in and causing havoc!!!,although I have no signs to date.)
Peril of country living.