Hello,
You did a great job on the sub box, have you thought about spraying the box with the stuff they spray in the back of pickup trucks for liners? It would be durable and you can get it in several colors like black, gray & etc...... I have a 2001 X-5 with a Non DSP system and I am going to use a factory subwoofer from an X-5 with DSP system that I bought and install it in my vehicle. The approach will be similar to yours except you fabricated your box and I am using the OEM Sub. Would you please forward me the color codes of the wires you used and perhaps provide some close up picture of the wires you used to tap into? Did you use the high level output from the Stereo Head Unit to the Amp or did you use the wires going from the amp to the speakers? Also, did you use the wires from the front speakers or the rear? I wish they sold a aftermarket audio cable and connector that was a plug and play unit for a Non DSP stereo to hook up a amplifier and subwoofer, but I have not been able to find one for Non-DSP. Please advise.