I need to exactly the same on my 203 c class coupe and have finally found the easy way (thanks to Richard).
Slide the drivers seat back & remove the plastic panel (which is held in by 3 torx screws)
that is above the pedals. Now looking towards the front of the car & upwards you'll see a grey coloured padded piece of sound proofing gently pull this down & you'll see a dirty yellow coloured piece of foam in rectangular hole about 100mm x 75mm, pull this foam out completely & you'll see a mass of cables; these are the cables that come own from the fuse box (drivers side rear of engine bay), carefully poke a long cable tie through the hole . If you now remove the fuse box cover you'll see the cable tie has entered the engine bay, tape the end of your new cable to the cable tie & simply pull it through into the engine bay. It's easy honestly but laying on your back looking up & under the pedals is just a little uncomfortable.
BTW Why are you running a cable from front to back ? I need to do it to fit an aftermarket set of parking sensors
Dave