Another known weak point on the E36 & E46 chassis are the rear shock towers. BMW has always tried to design the 3 Series to be a "lightweight" coupe or sedan and with that comes some faults. The rear shock tower in this instance is created using thin sheetmetal in the rear wheel arch. Unfortunately over time, that metal becomes weak and starts to "mushroom" or, in a worst case scenario, completely tears out and pushes the shock mount through the shock tower and into your trunk.
Installation of this reinforcement is very simple and can be handled by your weekend DIY'er or a professional. Either way, this is inexpensive part will give piece of mind whether just driving your vehicle on the street or tearing up your local track.