As you know, years ago Da'Lan made a tow hitch specifically for the 318ti, and then for some reason stopped making it. Since then it seems that every other Ti guy out there is looking for a hitch with no luck. I've seen numerous threads of people asking for a hitch option without anyone presenting a solution . . . Well, here it is.
It turns out that the Curt Z3 hitch # 11748 is a perfect fit! I ended up ordering one off of ebay for about $257.00.
http://www.curtmfg.com/part/11748 I originally expected that I would have to modify it in some way but it ended up fitting perfectly! Here is a copy of the installation instructions to help explain the installation steps:
Installation is pretty basic: You'll want to loosen each rear torx bumper bolt from the underside of the left and right bumper shocks. The bolts should drop straight out. Then you just slide the bumper back to allow you to get each horizontal bumper strut (as illustrated in the instructions above). You really do need to do this in order to get any tooling up there at all. From this point its easy, just set each end of the hitch on top of the bumper strut bars, slide the u-bolts around the bumper strut and through the hitch, secure with the nuts, and put the bumper back on. The only tricky part of the install is attaching the tail bracket to the trunk cross beam. There is no internal access through the top side of the trunk to get inside the cross beam (like whats pictured above). Instead I drilled holes all the way through the bottom of the cross beam and through the trunk into the car. I used my own 4" bolts to pass through a small 3x5 steel plate I used inside the trunk as a washer, through the cross beam and out through the underside of the car. They I used 2 more regular washers and lock nuts on the underside of the car. All in all I'm very happy with how it turned out. The hitch seems plenty sturdy and will allow me to easily pull my race trailer to the track.
Now everyone can stop hunting for one and just go get one.