i was poking around and found this black box disconnected. it lives under the front passenger "floor board". the part number is 126 820 08 26. i have searched around but could locate a description. i would like to know what it is before plugging it back in.

 

That item is the factory security system "brain". Its probably unplugged because of a malfunction somewhere in the system.

 

thanks. probably not worth trying to figure out the faults. i doubt it had keyless entry?

 

The W124 never received a keyless entry system in the US, the security system was armed/disarmed by turning the key in the door or trunk lock.