The speedo cable on my 70 280 SL enters the firewall between the connection point for the trouble light (small, black hinge covered connector) and the heater core connection. There are two grommets, one over the other. The top grommet has several items through it, including the tachometer cable. The one directly below is where the speedo cable goes through.
For access, the speedo is a bit of a pain. When I have done this for speedo repair, I took out the tach, which is reasonably straightforward, (it helps to take off the steering wheel - taking out the driver's seat is also a help, but not essential). Then I had enough access for the speedo.
You should be able to use the main search function to get some further information. This is a fairly common item.
