newb problem with bmw e90 trunk neg trig

So I am DIY installing my Viper 3303 alarm system on my 2009 BMW 335d (e90) and I have a crazy problem with the trunk negative trigger (my alarm system is installed in the trunk so I need an instant on alarm there). I am not doing a full installation with all the lights flashing, and door locks actuation, windows slapping burglars and what not. I am only hooking up a motion detector (in the center armrest), a tilt sensor (currently secured right-side up to the top of the trunk lid, but there is a 10-second delay on this so I need the instant trunk trigger). I have the ground hooked up, the hot 12v hooked up, the switched-on line hooked up, I found a positive trigger in the dome light door monitor, so the door monitor is working fine. I found a negative trigger wire coming out of the lightbulb on the inside of the trunk lid (I tested it and it has no power or ground when the trunk is locked, but when the trunk is unlocked, it has ground. There is also an always on 12v wire going to that bulb.) So I wired the negative trunk trigger from the viper 3303 to the neg trigger wire (soldering it onto a stripped section of the wire and covering the joint with heat shrink tubing). So now when I alarm to car, I get an instant trunk open alarm warning on my 2-way remote, but then it behaves normally. Then later on (time seems to vary) the trunk causes a full alarm. When I remove the trunk trigger wire, the alarm works fine. I am at a loss, no idea what to do except maybe add a resistor to the alarm line, I already tried a diode but that did nothing. The Viper trunk input is default factory set to "Normally Open", I can program it for "Normally Closed". But I dont think that is the problem because I think it would not work at all if it was actually "Normally Closed" so I havent tried setting the trunk trigger input to "Normally Closed". Ideas? Use my own pin? I am avoiding the CANBus adapter for monetary reasons, so that is out of the question for now.

Where is the switch that tells the light to turn on? You may have better luck going directly to the switch instead of the light.Kevin Pierson

1N4004 diode(s) on the POS (+) feeds RED / yellow to the trunk dome light(s). It's a spike induced by the BCM.

Great quick answers. As far as finding the actual switch. Well, it must be in trunk lid, it would be good to get into the lid and test the wires there. I know it is there because when I close the trunk lid latch by pushing up with the round barrel of a philips screw driver (spoofing the connector on the body of the car) and the light goes out. That would be the ideal solution, but digging into the trunk lid isnt something I am looking forward to. But a diode on the positive lead? Never thought of that. I think I will try that first because it is 1) easiest and 2) I feel like I am improving my car, and 3) I have the spare 1N4004 diode sitting around all soldered into the line I tried to use on the neg lead off the light. :). Thanks, for the quick responses I will let you know how it goes.

This was mandatory for me on the E46, the first one I ever did triggered the alarm, I kept getting trunk as the trigger. The E90 body wiring is identical to the E46, on both models the trunk trigger is a WHITE/ brown wire NEG (-), the trunk release is a grey/light green POS (+) relayed off your trunk release wire, 5amp fuse. Kevin, FYI the release solenoid and the trigger are in the lock mech in the trunk lid, BUT both wires are in the right hand floor loom (yes Kevin, our R/H/Drive and your L/H/Drive are the same). Look level with the rear of the front seat. Too many wires for amp and seat movement more up front. OP now you know this you can move the alarm control back up to the front and give yourself an ignition feed, bags of room beneath radio or driver side under dash! You don't need any CAN conversion on this vehicle if you're not doing an R/S, you can hard wire everything. P.S. Is there one trunk light or two on that vehicle? You will need to diode both if two. Right now I'm guessing lock and unlock colours but WHITE/ black for lock and blue/red unlock, both NEG (-) in loom in driver kick panel coming from driver's door, extend the lock pulse for comfort close.

Just one light in the trunk/boot. Yes, that diode in the +12v line seems to have done the trick, no more falsing from the trunk. :) However, occasionally when I arm it I get the message that the trunk is open (I also have the 516L I am trying to integrate). But the car doesn't go into alarm mode and it appears to arm perfectly. The weird thing is that it doesn't happen all the time. It's kinda embarrassing when you car basically tells you that you are an idiot in front of everybody. :) It is just one of those things which, over time, I may see a pattern and figure out what is causing it. Great advice and an easy fix.

Set your alarm to ignore interior light fade/delay. Or if you can access BMW diagnostics it will probably tell you it's a faulty trunk switch! I've seen this since on BMWs since the mid 90s, heck I have a Rover 75 Tourer which is effectively a 98-05 front wheel drive 5 series with exactly the same problem. It's a microswitch inside the actuator that starts to fail.

I have the OBD 2 adapter and the software installed on my laptop to code the car. I verified once that I could access it, but have never done it because of cold feet (scared I might do something wrong and end up at the dealer with an expensive reprogramming job). Well, I guess this is another reason to bite the bullet and learn to code my car. :)

Just a final post to note that the diode in the positive lead to the trunk light worked great. :) My only problem now is minor -- the 516L voice module confuses the trunk/hood violation and door violations, but the main unit doesnt. So when I set off the door alarm (I have the Viper 3303 violet + trigger input hooked up and diode protected to a lead off my dome light which goes hot when the light is triggered) by opening a door, I get the proper door open warning on my remote, but the voice module audibly reports "door violation", then "trunk or hood violation", then "door violation". It does the same thing when I trigger the trunk, but I in reverse order. This is not really important and not why I wanted the voice module in the first place. One final note, I found another thread here https://www.the12volt.com/installbay/forum_posts.asp?tid=120851&tpn=1&PN=1with a link to an installation manual which explains the programmable features of a similar DEI alarm. I highly recommend it for any other DIYers. This place is great. :)