Intrepid's step by step installation of a Dimmer to control Pressure in a Gaggia Classic.
For stripping the wire I used a pair of needle nose pliers with cutters. I gently used the cutters to clear off the wire. To 'crimp' I used adjustable pliers set on the smallest setting and just squeezed really hard.

First, I cut two 3ft pieces of wire. Stripped one end of each wire. Attached a male connector on one and a female on the other.

Next, I went to the dimmer. Attached a male connector to one wire and a female connector to the other.
I did not use the ground wire.

I than plugged the appropriate connections together.

I did not strip and attached the connections to the other side of the wires yet. I did not do this because I wanted to make sure the length of the wire is appropriate. Also, I wanted to run the wires through a vent slot on the back of the machine, the connectors would not fit through it.

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Next, I opened up the machine. Make sure to UNPLUG first!

With the machine unplugged and the top off, I searched for the wire I wanted attach the dimmer to. You want to find the wire that runs from the brew switch to the positive connection on the ULKA vibe pump. I used the wiring diagram for the classic, provided in the reference library here, to find the wire.

The diagram made my head spin. Sifting through the wires seemed to work better for me. YMMV.

After following wires around I found this was much easier than I was making it.

There is a convenient connection (shown in the picture below, I am pointing at it) that you will want to unplug (unclip) and attach the connectors from dimmer. I call this a convient connection because it clips and unclips easily and it is the perfect place to modulate the power to the pump. This convenient connection runs from the brew switch to the positive end of the vibe pump.

Edit: In the picture the wire colors are red on the left and white on the right. Information added due to question asked later in the thread.

Your wire colors maybe different. The color of the wires will depending on the year of the model.

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From here I fed the wires I attached to the dimmer through a vent on the back of the machine.

I than attempted to stage where I wanted the wires to be place when everything is done. This allowed me to get a sense of how long the wires should be. I found that 3ft was a tad long and cut off around 6 inches. YMMV. This will depend on where you want your dimmer located.

Now that the length was where I wanted it I stripped the wires and attached the a connectors that opposed what was already attached. Meaning, if on the other end of the wire there was a male connector, I attached a female, vice-versa.

Next, I disconnected that convenient connection and plugged in the appropriate wires from the dimmer to the convenient connection.

I fed the wires from the dimmer through a zip tie I had already placed in my machine from when I installed my PID (see picture). The zip tie helps keeps the wires in place and keeps the inside of the machine looking cleaner, well to me at least.

Edit: To avoid confusion, I am pointing at the new connections I made in the center of the picture. I noticed that it may look like I am also pointing at the red wire on the top left of the picture, please ignore my other finger, as it has nothing to do with the dimmer.

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Final step. Testing!

Not going to lie here. I was not 100% sure if this convenient connection was the wire I was looking for. I did the best I could tracing the wire and looking at the diagram but it was slightly difficult. Luckily, everything worked out!

I used the pressure gauge I made to test and get a feel for how well the switch works.

I found when the dimmer was all the way on (slide pushed all the way up) I was getting 10 bar! Excellent, that is what I adjusted the pressure to a couple months ago.

Playing around with the dimmer, I ended up making marks for 3 Bar and 6 Bar* (see picture). I did 3 and 6 Bar because those are clearly marked on the gauge I have.

Right now I just have the dimmer sitting to the right of the machine. Not sure where I am going to place it. If I really like the shots I am getting with the dimmer I am thinking about making a box to mount it in. Possibly mounting it on right side or on top... We will see. 

*You may notice the 6 kinda looks like a 7... Well it was a 7 at first. I first wrote a 7 because that was a guesstimate for the first dimmer adjustment I tried. I later decided that I would rather mark 6 Bar because it is clearly marked on the gauge and 7 is not.

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