I also ordered the OEM sub from ShipSound--no instructions also. I had it professionally installed by Mickey Shorr (ABC Warehouse) for $75. Best Buy also quoted $75 for the install.
It took them less than two hours. It was worth to me since it probably would've taken me a whole day given my technical ability!
I thought about going to Best Buy, but would they install it how they want to install it to limit the number of hours of installation time, or would they do it the right way (drilling to secure box, running wires through the panels vs under the car)?