There were 2 main reasons for me. Crevier BMW (my parts supplier of choice in CA), was not selling BT ULF modules due to the fact that BMW USA still had >3,000 early generation modules that they were forcing dealers to sell before they would let dealers start shipping the latest ULF modules. AA's had the ULF module in stock at roughly the same price as going through the dealer. In addition, AA's makes the needed custom wiring harness that connects to your DB25 plug in order to make the system operational. Martin Bishop is the only other person to get this custom harness from. The downside is that AA's is a small outfit, very busy, and often tough to get information from. I'm pretty sure that even if you get the module from the dealer, that you will have to buy the custom harness to make it work.