I've had a 1990 240 DL for about 5 years now. 2 days ago someone opened one of the rear doors (which was unlocked) and set off the alarm. I didn't even know the alarm existed. There was blaring siren, which after a few cycles went off but the alarm still appears to be activated, opening any of the doors sets of the siren. The little red light to the left of my steering wheel has gone from always on to flashing and I can't start the car.

I can't unlock the car from the driver's side door because someone at one point attempted to break in with a screwdriver or something so I have to use the passenger door to unlock the car. So if all it takes to disable the alarm is to use the keyhole on that side than I'm S.O.L.

I've tried disconnecting the battery and have also looked for some kind of alarm switch (assuming this is an aftermarket alarm) to no avail. I seem to have no documentation about the alarm (and no way of contacting the previous owner.) I've read the manual multiple times but can find nothing. I've replaced fuses as well.

If anyone has any insight that'd be great. I just want to find the cheapest way to make the car work again. If that means disabling the alarm that's fine. The car's old and falling apart so I tend to leave it unlocked anyway. I don't want to have to tow it (or pay a mechanic,) but unless I can find a way to get it started in the next few days I'll have to.

TL;DNR I'm pretty sure there's an aftermarket alarm in my car and I have no idea how to disable it.

EDIT: I just found a toggle switch behind my fuses. tried flicking it an starting the car: nothing. the alarm sounds very weird, quiet and strained. When the key is in the on position the alarm goes off and all power to the car seems to cut out.

 

i had the same exact with my 87 760. all i did was disconnect the battery, leave it disconnected for about ten minutes, and all is fine now

 

I've tried doing this about 50 times. No luck.

 

I would look under the dash and find a original wire that has been cut and some sort of box spliced in. Re-connecting this wire back together should bypass the lockout. Then cut the speaker wires or remove power to the alarm box.