Ignition switch doesn't turn off

I have a 75 450SL with what I think is a faulty ignition switch. When trying to start the car, the starter keeps turning even after turning the switch off. I would like to replace the switch myself, but have been unable to locate a detailed drwing for removing the dash cover to get to the ignition switch assembly. Any advice on where I can find such a drawing and is the ignition switch problem a common problem?

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I haven't done this personally, but saw my mechanic remove the instrument cluster (dials) from my car to get at my ignition switch. You must first disconnect the negative battery terminal cable at the battery.
He told me to pry the cluster (carefully!) out around its periphery, which is exactly what he did - (no screws or clips). The instructions I have seen for my 450sl say the same, and that the cluster is held in place by a 'rubber section'.
To get to the switch it would seem that you will need to disconnect the speedo cable and oil pressure line from the back of the cluster (or is that 'front' of the cluster?), plus some electrical connections. (Instuctions are on another new post nearby, but I'm not sure how to link you to it......)
Sorry about lack of picture guidance.
Good luck,
Daniel.