Christian,
it wiggles in sideways from the passenger footwell. Not easily, but it'll go eventually. Start with it in approximately the final installed orientation, and then you just have to fuss and twist and fiddle until the right position(s) are found for it to slip in. I'll bet after the first couple hundred, the guy on the assembly line could do in less than 10 minutes.
George Davis
'69 280 SL Euro manual