[W126 Coupe] How do I know which control arm is for my car?

[Shayegan, Richard]

When I scheduled an appointment to have the mechanic replace my left rear control arm, I told him I would bring my own used part. He said to make sure it was the right one for my car or I would have to swap out the brake cables. Not wanting to sound stupid, I said "right" So....how do I know if I'm getting the right one for my car (1985 380SE, after the VIN break).
Thanks,
Richard