MK IV dec - speedo cable

How exactly goes the speedo cable from the gearbox to the speedometer? does the cable stays under the floor and behind the firewall? or is the cable coming inside the car under the carpets?

Under the floor and then up inside the firewall

This is a description for a right hand drive. I am not familiar with the LHD.

From the gearbox, the cable passes through the large holes in the transverse chassis rail, stays under the floor board and emerges through a small hole at the front edge of the floor board beneath the steering column, against the inside face of the bulkhead. From there it lies flush with the bulkhead, staying vertical, before turning horizontally to track just under the scuttle panel to the back of the speedo.

Just a few hints - take care with the section under the floor board as it will be exposed to exhaust pipe heat radiation and is close to the pedal arcs, and the last section at the top should be routed around the bulkhead stiffener in the slowest curves possible.