Stellar (autoloc) makes a wide range of products that utalize the factory wiring and alarm port for less than the factory setup. I recently wrote a DIY on a few topics, and keyless/alarm install was one. Tells you how to get to where it is and shows you what it is you're plugging into. The topic is in the audio forum.
<span style='color:gray'><span style='font-family:Courier'>1995 e36 3 Series RIP
1999 e46 3 Series BURN IN HELL
2000 MR-S Roadster= Current</span></span>