Last week I bought a used Uniden BC890XLT scanner and added a discriminator audio tap. Here's how I did it:
The first thing to do was open the case. The outer sleeves come apart on the top and the bottom. There are 2 screws on each side and one screw on the back that I removed. I took both the top and bottom sleeves off to explore the inside, and discovered I only needed to remove the top to tap the discriminator audio. Once inside I found the location of the discriminator audio which is pin #10 of IC #3, 3359DA. Pin #10 is on the top right corner of the chip:
Rather than solder to the chip, I preferred to solder to the lead to the resistor at TP3 which is connected directly to PIN #10 of the IC. I put a 10K ohm resistor in series with a 10 μF tantalum capacitor.
I wanted this modification to be removable so I didn't drill a hole in the case. Rather, I ran a connected wire to the outside of the case through an opening in the side of the case.
I then ran the wire in the space between the case and the outer sleeve on the side. I then ran it around the corner to the back of the scanner. I connected the ground wire to the chassis via the screw that holds the accessory RCA jack. I put heat shrink tubing on the outside of the wires and connected them to a 1/8" mono phone jack.
Radio Shack Part Numbers:
1/4-Watt 10K ohm resistors - #271-1335
10μF Tantalum Capacitor - #272-1436
1/8" Mono In-line Phone Jack - #274-0333
Assorted Heat Shrink Tubes - #278-1627