Vintage Cars: 1979 mercedes 450 sl, lever arms, voltage meter

Question
Ron, I have a 1979 Mercedes 450 sl and both brake lights are not working. Have checked bulbs and they are ok. Have check the ground and it seems to be fine.  With both bulbs removed, and the car in reverse the voltage meter shows 12V. As soon as one bulb is put in, the voltage drops to zero. I though it had to be the ground but is seems ok and all the other tail lights are working and share the same ground. Is there some type of switch that gets engaged by the shifter when it is put in reverse? Any help on fixing this would be greatly appreciated.
Thank you,
John

Answer
yes John , there is a netral safety switch, the contacts can sometimes get gummed up and not allow for the reverse lights to come on. If you decide to work on it ( it can be cleaned ) pay careful attention to the orientation of the lever arms


http://www.mercedesshop.com/Wikka/W108NeutralSafetySwitch

this is a 108 repair, but I believe the 107 is similar.

you mention brake lights and then back up lights. Brake lights also have a "switch" so to speak , on the arm of the brake pedal.

Also make sure the wattage of the bulbs is correct and check for a short in the bulb base