1993 240 sedan: repaced both brake lights,rt side bulb opaque white,lt side looked as if it exploded. replaced with identical replacements, now the confusing part.if car is shut down completely, pushing the brake pedal down you get a normal turn on of all 3 back brake lights. if the ignition is turned on and the car is running, brake tail lights are in a dimmed mode, pushing the brake pedal turns off the tail brake lights and turns on the brake light in the rear view window. ???
i checked fuses[ok] was wondering about relay switch, would appreciate help.
thanks
bill k

If they work normally with key off, the circuit is all just fine.
Probably the problem is happening when the parking lights are on, which is the case in the 86+ cars when you leave the headlight switch in the on position- a lot of people do this including me.
The brake light bulb shares a socket with the outboard parking light. Note that the two sockets are different left-to-right and different from all the others in the housing. The inboard parking light is a totally different bulb and socket. The white socket should be on the right outboard position, too.
If you've pulled a white, cloudy, yellowish, or cracked bulb out of either brake light, the problem is water intrusion. I've "fixed" many taillights with this problem by drilling a small (1/16 or so) hole into the bulb chamber to drain out any water. This at least saves the bulbs and extends the life of the housing for years, potentially. The only real fix is new taillights, which is big $$.
One last thing not to be overlooked, is to check the ground wires. The black wires connected to a screw on the rear bodywork need to be clean, shiny metal. Clean rust off the body area and the terminals.

Good luck!
--
Rob Bareiss, New London CT ::: '87 244DL/M47- 230K, 88 744GLE- 220K, 82 245T-181K Also responsible for the care and feeding of: 88 745GLE, 231K, 87 244DL, 239K, 94 855GLT 189K