Does anyone know how to disassemble this? It's completely worn out and means that when the motor cable attaches, it just spins inside and doesn't get any traction. I have located a rail from a W219 that has the same gear and so if I know how to disassemble this one I can swap them over.

Try to get the worm screw freed out from one end. If impossible it's a non serviceable assembly.

Front seats are integrated to SRS and move at high speed during collision impacts.

Keep that rail mechanism in good repair order.