Hello wd,
Assuming that you have the early style taillights (the "all red" taillights) there is one "bulb holder" = taillight back housing for all bulbs which is made of zink-plated sheet metal.
Please see our wiki for the details:
http://www.sl113.org/wiki/Electrical/TailLightsOnce the copper contact points for the single bulbs "break" off, the only way is to "repair" that somehow or get another used (or NOS if you can find one) complete bulb holder assembly. It's mostly tricky to repair them but not impossible.
Unfortunately, no official spare parts have ever been available for the bulb holder assembly.
Basically the same problem applies to the bulb holder of the late style taillights.
Please make also sure that the ground connection is in good working order, which is a brown wire coming out of the electrical 6-prong connector that is attached to the chassis. See also the wiki (link above) if you are unsure about the layout.
Bad ground gives you all kind of different weird blinking/fading effects of tail light, back-up light and turn signal light.
Good luck & keep us updated
Achim
(early style taillight collector - all garbage)