To replace mine (124 250D right hand drive) the rear seat back had to be removed, 3 bolts along the bottom edge and lift a little to unhook it at the top.

The Coax comes thru the rear bulkhead, under the rear seat and into the plastic wiring trunk that runs along the cars floor, then up to the radio under the carpet at about the same fore/aft place as the radio. The front seat secures the carpet down so that has to come out along with the clip on trim at the bottom of the front & rear doors, easy to do, it just takes a little time, the opportunity to give it a good clean out in the places you just cant access shouldn't be missed.