Overview
This is a simple little hack to extend a few of the keypad buttons on a Radio Shack PRO-2042 scanner. This will allow a person to activate a button on the scanner remotely, without having to be anywhere near the scanner. This is very useful for those crowded SIGINT environments where you may have a lot of radios going, and you don't want to get up all-the-time to press a "SCAN" button.
This example remote is for the Radio Shack PRO-2042/PRO-2035-series of scanners only. Other Radio Shack models will have a similar keypad interface. The keypad schematic for a Radio Shack PRO-2006/PRO-2005/PRO-2004-series scanner will be posted at the end of the article.
The actual remote hardware is quite simple. All you need is four 47k resistors, some 1N4148 or 1N914 diodes, push button switches (normally open), wire (8-conductor CAT-3 is best), and some male/female DB-25 connectors.
Schematic

Operation
The remote's operation is quite simple. It's just the scanner's keypad signals routed to an external push button. The resistor and diodes are for isolating the signals from the other keys. Each wire going to a column (COL) line needs a series isolating diode. This is very important for isolating each of the keys. Look at the schematic to see an example of multiple wires going to a single column. The row (ROW) lines each only need a single 47k resistor to ground.
Example: If you want to extend the SCAN button on a PRO-2042 scanner, you need to connect both the ROW2 (pin-2 on the PRO-2042's front-panel keypad connector) and the COL1 line (pin-5 on the PRO-2042's front-panel keypad connector). These can be connected together via a simple push button switch.
PRO-2042/PRO-2035 Keypad Matrix
The numbers in parentheses () are the pin numbers on the front-panel keypad connector. That large, white 15-pin connector behind the scanner's front-panel. It's labeled CN501 or CN503.
COL1(5) COL2(6) COL3(7) COL4(8) COL5(9) COL6(10) COL7(11) COL8(12)
ROW1(1) MAN AUTO RECT LIMIT PRI 1 2 3
ROW2(2) SCAN PGM L/OUT UP MODE 4 5 6
ROW3(3) TUNE TX L/OUTRV DWN STEP 7 8 9
ROW4(4) SNDSQ ENT DEL MON RESET 0 . CLEAR
Example: If you want to extend the MODE button on a PRO-2042 scanner, you need to connect both the ROW2 (pin-2 on the PRO-2042's front-panel keypad connector) and the COL5 line (pin-9 on the PRO-2042's front-panel keypad connector).
Pictures

Picture of a Radio Shack external speaker (Part Number 21-549A) which contains the remote control's SCAN, MANUAL, UP, and DOWN buttons. A 100 ohm potentiometer is mounted on the side of the speaker for use as a secondary volume control. (Wire it like a variable resistor, in series with the speaker's + lead). A male DB-25 connector and the speaker's 1/8" plug are shown on the bottom left (they come out the back of the speaker). These connect to the PRO-2042 scanner via about eight feet of cable.
(Don't be a dumbass and break the plastic when drilling...)

Internal speaker picture showing the incoming wiring (bottom) for the four push buttons and the volume potentiometer (right).

Close-up internal view of the speaker. The big RED wire is the speaker's positive (+) lead.

Internal view of the PRO-2042 scanner. The resistor/diode interface network is on the lower left. It routes, via SIP sockets, to a female DB-25 connector coming out the back of the scanner.

Close-up view of the resistor/diode interface network. The interface's connections are via SIP sockets for ease of disassembly. The SIP sockets are not required, but highly recommended. The RED arrow points to the large, white, 15-pin, front-panel keypad connector. The top pin of the connector (looking down) is Pin-1. The metal shields can be soldered to for ground. You'll basically have to take the entire scanner apart to reach everything.

Alternate view. There is a notch cut in the front-panel's metal shield to allow the wires from the keypad's connector to come out. There are small holes (and solder pads) near the connector which you can solder to.