7 years later, the battery was dead. No more charging. I found several guides on how to change it, and now it's like new again. While looking inside, I realized there was one thing missing: The thumb button I used to love. So I started planning. Sadly, I didn't take pictures while I was working, but I have some pictures that prove the concept, and a few things I had to deal with. I also have some plans for making it better.
Opening the mouse it pretty straight forward, four screws, two under the label, and two under the back pads. There are lots of guides on how to do this, and I'll put up some links later on.
The MX1000 is mostly seperate modules inside: The scrollwheel and buttons, the sidebuttons, the optics and the battery. I was interested in the sidebuttons, so first I removed the battery, as it is blocking the access. Next I pulled the tap back, and the button pcb slides out. On this pcb there are 3 buttons: Back, Forward and task selector. I use the back-button all the time, but because of my short finger, I never use the forward button, so I decided to piggyback on this one. I solder on the wires, and didn't measure any length. This came back to haunt me later.
Now it was time to make room for the button. I drilled a 4mm hole at the place the thumbrest was most worn, as I figured that's where my thumb was the most. Bingo, perfect placement. I then filed it to a square, so the regular pcb button would fit snuggly. I tried several different buttons to find the correct length, and ended up with a 7,2mm heigth. I didn't glue it in place; as I plan to replace it with a better button-top later, I just pressed it in for now. Soldered the two wires to the forward-button, and assembled the mouse. Testing worked, but the scroll-wheel didn't work for the left/right-clicks. Dissasembling the mouse I could see the wires were in the way, so I cut them to perfect length, and now it all worked.