Do you think the problem is really with the BCM??
Took my 99 300m to the dealer because both key fobs stopped working. Thought the batteries had died. The dealership is saying the problem is with the BCM and it needs to be replaced. I can lock/unlock the doors by putting the regular key into the door lock. If I turn the key once, it unlocks the driver side door, if I turn it again, it unlocks all other doors. Same for locking the doors. The red security light comes on when I use the regular key to lock the doors. I can unlock and lock doors with the door switch on the driver's side door panel also. I asked the dealer could I go to a local store and just buy a replacement alarm system/key fobs. The dealer said that it would still need to go through the BCM. I called the local Best Buy and the tech said that since I can use the door switch to lock and unlock doors, he doesn't think the problem is with the BCM and they can add a new alarm system/key fob. I tend to believe him since I am not having any other problems locking/unlocking the doors. I just can't use the key fobs to unlock/lock the doors.
If I were you I'd just have Best Buy install the new system.
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