(Scroll to the bottom where the pictures are)
http://www.atariage.com/forums/topic/13 ... ble-packs/
& a video
http://www.youtube.com/watch?v=yEn6LT4DUNA
I figured it would be possible with the tremor pack plus (the only rumble packs I have). I did some Google searches and didn't see any other battery-free mods done with a third-party rumble pack. I saw some people that asked if it was possible with any rumble pack and mentioned that the controller receives power from the console so it could be taken from that, but I didn't see anyone that did such a method. The mods I've posted move a resistor from one point to another.
I decided to go ahead and perform the mod with my tremor pack plus anyway (Thanks to skyone for the controller pinout and this forum that keeps it posted). I simply wired the V+ that normally attached to battery terminals to the 3.3V line that goes into the rumble pack from the N64. It works great; I used it for 30 min. Theoretically it would be more powerful than two batteries that equaled 3V~, though I haven't compared.
Here's a picture of the mod:

My ultimate question is do any of you see anything wrong with this mod? The "common" mod involves a resistor and on a page I looked at that dealt with the mod there were naysayers (here is the page: http://www.assemblergames.com/forums/ar ... 21639.html). The naysayers stated the N64 power supply wasn't meant to handle the load. I say it is an insignificant load, as did others. Since this forum deals with the N64 power system the most, I would trust its members to have the best opinions.