The Delphi guide rod is the leading control arm in the E90/E92/E93 M3 rear suspension (and E82 1M). It sits halfway up the wheel carrier in between the trailing arm and upper wishbone.
A control arm should be replaced when the ball joint or bushing shows signs of wear. This control arm uses a rubber bushing on the outside that will affect the alignment if it's worn out.
Left Side = Driver's Side
BMW Fitments:
1 Series
E82 / E88 (08-13) > 1M (11-12)
3 Series
E90 / E91 / E92 / E93 (06-13) > M3