[timn]

Has anyone been able to install a battery backup for the OEM alarm?

A very common battery backup used is the DEI 520T.

It requires these connections:

1. +12v battery
2. +12v alarm
3. alarm trigger
4. ground

#1 and #4 are easy.

The bigger question is #2 and #3.

I'm guessing that #2 can be worked around by wiring the siren's +12v, that will at least get some noise out there. Does anyone know if the siren's +!2v wire is easily accessible from inside the car (like in the trunk or hood area?)

For the alarm trigger, in the DEI shock sensor DIY, the trigger wire is found in the rear dome light area. Same question as above, are there any trigger wires we can tap into in the trunk area or hood area?

[SMOKE EM IF U GOT EM!!]

i know the connection for the siren and the mount is in the left rear wheel well! you have to remove the cover to access it, if that helps

[timn]

I'll look into that this weekend

Does anyone know where I can get wiring diagrams for the 335i?

[ScotyH]

I'm pretty sure the factory alarm has a Ni-mH battery in the siren already.

[Placey]

It does have one built into it already