The discriminator mod for the PRO-2045 is very simple to perform, everything you need to perform it can be purchased at your local Radio Shack or any other electronics store.  As long as you dont have the ctcss board installed you can just tap the signal right there, no soldering needed.  If you do have a board installed you should be able to tap the signal from pin 9 on the mc3361 chip.

     Here is a step by step installation using the ctcss board plugs.  Click on each picture for a larger view.

1. Remove the 3 bottom screws and 2 top screws on side.
3 screws  2 side screws

2. Carefully slide back the top half and pull it up from the rear.  The speaker is
still connected to the board so ease the top off.  You can remove the speaker
connector for easier access while working on the tap.

3.  Now for the tap.  I used 30AWG wrapping wire from RS, you can use any
wire that fits though.  Push the stripped wires into the middle plug of the top 3 
and 2nd plug from the right on the bottom four (ground).  Depending on the
size wire you use, you may want to bend it before you push it in so that it makes
a secure connection (should stay in when you lightly pull on it).
click on either image for a closer view

4.  Next you need to select a location for the jack.  I used a Dremel to drill the 
hole for mine, remember that its better to have the hole too small at first than 
too large (easier to make it bigger than to make it smaller).  Use whatever jacks 
you want, i used a 1/8" phone jack (RS part # 274-248A) positioned left of
the bnc connector on the rear panel.  After the hole is drilled solder the wires
to the jack (wire from the 4 plug line goes to ground) and install the jack with
the provided mounting hardware.
1/8 jack  open rear view

5.  Now close up the case and start monitoring those digital signals.