I put bass in the trunk of a 1998 328i using memphis car audio componenets. use a .75 cubic inch ported box w/ a 10 inch sub single voice coil, fire toward the rear of the vehicle. Powered with 75watts X 2RMS/ 300 watts memphis amplifier part number 16PBC300. Amp is fan cooled so no need to worry about overheating. Ties in before the stock amp located on the left hand side of the trunk. No need for other modifications unless you want to blow the windows out. This set up works nicely. cost me $400 installed. Bass is non directional and travels through steel, no need to vent into passanger cabin. Hope this helps!