Hey techs out there I seem to have a bit of an electrical problem with my 300e Here,S how it goes. The radio,The clock, The inside door lights(the lights when you open the door)The lights on the window switches(when the park light is on)an the vanity mirror lights don,t work when the front passenger door is closed.When the front passenger door is open all the above work fine, Except the Tach which stops when the passenger door is opened. Now I recently installed a Prestige Car alarm in the car, now the alarm seems to be still operating fine, but could this be the source of my problem? I know this is kinda confusing electrical problem but this kinda stuff can only happen to me.

1987 300E

It sounds like you have a problem with one of the ground wires in the system... Check the connections you have made for your alarm; there is probably something wrong with that (I would say...).
Start with a check of the alarm-wire connected to the door-contact and the other ground wires of the alarm (if they are connected to an existing MB ground wire instead of a grounding point).

The inside door lights(the lights when you open the door)and the vanity mirror lights are (usual) plugged into the front courtesy light; check power and ground there.

Does it matter whether your ignition is turned on or off and do the power windows function when that light is shut off (I had something similar with my C250D and had a short in the electrical mirror adjusting)?

greetingz,

There is a small diode in the fuse box that allows the seats to operate with door open & key off. The diode is there to keep current flow from battery power with key off from feeding into key-on circuit, seat operation with door closed & key on. Sounds like this diode has "shorted" & is allowing current flow in the wrong direction. Small grey or black 2-pin plug in plastic device in the relay section of fuse box! Try un-plugging & see what happens!

M.B. Doc
 

Perhaps that explains why my seats operate with the driver's side door open (both key on and off), but not door closed (even if key on)?

DOOR closed w/key in is powered from fuse # 2 & then thru diode to relay. Either fuse or diode is bad!!

Thanks
 

Already checked/replaced fuse (even though it was still good, thought it might have microcrack). Will check diode next.