I put a powered headunit in and left the exisiting amp/speakers and i used the harness, not the RCA plugs. it sounded fine.. it just might take alittle work getting your headunit to turn the amp on. you could try the RCAs.. but i mean.. heh.. the factory amp is so awful that i'd hardly make a difference. whats the factory amp put out? like 20watts/max per channel? i forget, it might be less than that. but anyway, just try it both ways. buy a harness adapter for the e34, wire it to your headunit's harness, plug it and see how it sounds.. then either get lazy and leave it (my vote). or disconnect JUST the speaker wires from the harness and splice them to the RCAs.