Ranger, what you and your family are probably hearing is the gas filler door locking. If you are on the passenger side, you can hear it. That's what I use as a confirmatory sound.

 

Success!! ...Did it! ......i was able to follow the wire from the fuel door actuator to access it at an easier place .....if you look at the wires at the rear tail light there is one cable with a black wire and a blue wire coming down vertically ..the two wires are wrapped in a cloth-like wrapping ......i first tapped into the blue wire for a positive ...for the ground just below that is the 5 wire plug that goes into the tail light and the middle wire, brown, is the ground and i tapped into that. I bought a piezo buzzer 12V / 58 ma /100 db at my local Radio Shack which unfortunately ( fortunately for me though) is closing so the buzzer was 80% off so it cost me only about $3.50 ...!

it worked ...BUT ...opposite of what i wanted .....oops ...the buzzer beeps when you unlock the car ....( not as important to me but then again i may go back and get another buzzer to beep for unlocking ) ......but then I
disconnected the blue and stripped the black wire and tapped into that ...success ...the car will beep only when the car is off and closed and then you lock it ....it does not beep if you are in the car and lock it manually or if you drive off and the car " auto-locks" at 11mph ......

I simply ( with some added length wires) sticky velcro'd the buzzer so it is sitting on a little shelf-like area down below the tail light. The buzzer is noticeable and loud enough, though it could be louder but you can definitely hear it lock now ......

 

My M240i beeps on lock (not a horn beep) sounds like a computer boot up beep. Hardly a sound that will annoy a neighbor or anyone else.

 

just a quick follow-up ,,,,it has been a couple weeks now since i added the buzzer that goes off upon locking with the FOB ( no stock alarm in my car) .....no problems ,,,.....no interference with the car's electronics

 

Good job Umberto. Now there's another project I have to get around to.