It should be easy enough to track down. Since it is an aftermarket unit that will mean the the installer will have most likely used crimp-on or vampire connectors to wire the thing to the car. Once you've found the "brain", start tracing back the wires that come out of it 'till you find non-OEM connectors. Hopefully most of the connections will be of the vampire variety so you can simply remove the connector. If you encounter any where the unit was spliced into a wire (i.e. wire was cut and box put in the middle) then you will have to get a bit more creative.

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"It's not about how fast you can go, but how well you can go fast."

Bob in Richmond

'97 S320 (LWB), Ruby Red Metallic, 73k miles
'97 S420V, Smoke Silver Metallic, 155k miles