yep... it's a small investment to make for the security you get back in return... pays off even more when your 'theif trap' works. Kinda like setting up a booby trap for an animal.. you know sooner or later one will come along and try to take the bait. I think we've all realized it's only a matter of time before someone either steals something off our cars (i.e. foglights, emblems, rims, etc.) vandalizes our car (steals stereo included).. or steals our car. Of course, having your car stolen being the worst case scenario. So why not prepare yourself in advance for the inevitable. Custom work will always do a better job than a store-bought alarm system.
To add to my above post:
I previously had the idea of also double-locking the doors when they begin driving away... essentially trapping them in the vehicle unless they shatter the glass (difficult to do from inside the car.. no leverage!.. and try climing out of a car through the window... pretty difficult to do too.) Maybe put a few pizzo buzzers in there.. a good 30 seconds before their ears bleed... some nice interior strobe lights to disorientate them too. But then I figured... what happens if something goes wrong and it happens to me.
But all that is possible to do for under $50 and around 3 hours work to do a clean install.
I'll give out a few hints as well.. but I won't give out any secrets I use for my security.
1) put a momentary switch (similar to what you'd use for a start engine button) at the very very very top of your dead pedal. Paint it black of course. Wire this up to the ignition. Car won't start unless your foot is all the way at the top of the dead pedal.. when you move your foot, (after the car is started), the ignition will be disabled again. Why do this? Well you don't want to make a switch which you visibly have to flip/change when you have guests in the car... you never know who can turn on you, or how quickly information can spread. Secondly... most cars require you to hold the clutch down to start the car.. (manual of course)... however, this is not the case with BMW. Thieves will have their foot with the clutch down trying to start the car... their foot won't be on the dead pedal.
2) Use a REED SWITCH. This is one of the stealthiest ways to put any type of kill switch in your car. It's a magnetic switch which opens and closes the circuit whenever you place a magnet near it. Alternately you can get a reed switch which will keep the circuit closed (or open) for around 15-20 seconds before it changes back to its rest position.
How it works? ... place this reed switch anywhere you want. A nice place I installed this on a friend's CLK once is behind the B-Pillar... the owner has a magnet on his keychain. When he unlocks the door, he keeps his keys in his right hand as he pulls the door open.. the magnet dangling from his key chain invisibly flips the reed switch hidden behind the B-Pillar for around 30 seconds... so he essentially has 30 seconds to start his car. If you do this, keep a spare little tiny round magnet in your glovebox for emergencies... what thief will think of this? .. and how would they know where to put the magnet? You can put this reed switch pretty much anywhere.... keep it away from door speakers obviously (magnets), and close to the surface.. you don't want to use a huge magnet to get to the reed switch deep in your interior. I get my reed switches online... designed to the specs I need.. (how much voltage... default on/off position.. .and if it has a timer.. price? around 30 cents... usually they'll send you a free one if you say you're interested in buying more.. just need 1 for a prototype for some project you're developing.)
A good way to describe a reed switch with a timer is like the blinker-bulb on a set of christmas lights... the little switch closes and the lights come on.. the heat in the bulb splits opens the switch again.. when it cools, the circuit closes again. (and the lights go on.. until they get too hot.)
3) don't even use a switch.. instead attach a relay into another circuit on your car. For example.. your map reading lights... or your glovebox light. Unless those lights are on, your ignition won't work. One of the neatest kill switches I ever installed.. on a car I won't mention (not my own), is a fuel pump kill switch integrated into the car stereo. The stereo must be on at all times when driving.. (who turns it off anyway?)... the cool part is, when you leave your car you take your face plate with you... impossible to start the car. So you have to have your face-plate on in order for your fuel pump to work.
Other preventive techniques on BMW's...
1) disable the central locking for your trunk!...
you can do this by enabling the valet lock on your trunk. (turn your key two positions to the right). If your key can't fit in the trunk lock then you need to go to the dealership and get them to cut you a master key.. (which'll work for every lock on your car.. doors..trunk...and glovebox). Why do this? ... well if they unlock your doors, your trunk will unlock too. How long do you think it'll take if one person was waiting at the trunk, as soon as the car alarm goes off they open the trunk and cut the power wire from the battery? No power = no alarm. no alarm = tow it away easily... or rape your car of it's stereo and whatever else they want. The downside?.. you can only open your trunk with the key.
2) hide your siren. don't keep it visible in your engine bay.. or put a second hidden siren ($5) under the body of your car. It's just as easy to pop the hood and cut your siren. No siren, and your car is just flashing it's parking lights instead then.
3) I know you're thinking hey, I have a pager alarm! So I'll know my alarm went off anyway! ... wrong answer. It's just as easy to cut that antenna which sends a signal to your pager... especially if you leave your window or sunroof open. Likewise, if you have a FM pager (not spread spectrum), you can make a FM Shortwave Signal Jammer for around $8 from parts available at Radio Shack. All they have to do is double side tape this to the other side of your windshield where the antenna is... and your pager won't receive any message. Sucks to be you.
hope this helps. any other questions, just shoot my way.