Speedo cable question

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My 350SL has an after-market cruise control, the old fashioned style that was frequently used on GM cars of the mid-60s. The amplifier sits on the left front fender apron forward near the headlamp bulkhead. The speedo cable is routed from the transmission to the amplifier and then another cable runs in a reverse direction straight back toward the firewall. In order to reach the dash the cable passes through the firewall at a large diameter hole, about 2 inches, adjacent to the brake booster. This large diameter hole appears to have once been sealed off with a plug, but is now open for the cable and the old cruise wiring to pass through. The cruise control mechanism is partially removed and inoperative, but the amplifier and wiring is still bolted on. I would like to remove it and return the speedo cable to original and properly seal this large hole if possible.

The question is: What is the original and correct routing for the speedo cable?

My old 114 sedan has the cable running up to the dash from the transmission through a hole in the the transmission hump, near the gas pedal. It is fairly direct. Is the 350SL done similar, or is the long route up around the engine compartment, looping past the brake booster then looping in through the large firewall hole actually correct?

None of the manuals have photos that show this area clearly enough to see anything.

Help!

230/8