I would like to add my method of modifying the newer Belkin Tunecast 6 FM Transmitter for greater RF output power. There are several projects here relating to the Tunecast 2, but the Auto 6 model is much harder to get into - here is how I did it.
First use a small screwdriver to remove the two switch pads, if you slide the screwdriver along the upper and lower edges of the 'scan' pad it will snap free when you rotate the screwdriver, some determination is required here! Then remove the '+ -' pad using the same method, but near the center, you can see the mounting holes in the picture.
You should now be able to see the white antenna wire that goes around the display, hook this out so that it can be cut. Now drill a suitable hole in the case (careful not too far in!) for the new external antenna loop, about 65cm will do nicely. Now Insert the new antenna wire in the hole and solder to the cut ends of the old antenna wire. Use some epoxy to secure the new antenna wire to the case.
Snap back the pads and you are done! Mine will now happily overpower most stations, but don't be anti-social, use the auto-scan to find the weak ones!