Also, for others viewing at a later date, you can run the wires and whatnot through the fusebox on a 300SD (w126) as the fuse box has MANY extra holes of varying sizes and also, where it enters the cabin, has a HUMONGOUS hole that you can easly get things through the firewall.

Doing this means removing the two screws on the top of the fuse box and lifiting it up to reveal the rats nest underneath.

This is what I did on the SD to install a boost and pyrometer line into the cabin...worked out quite nicely