I haven't read the instructions for the Sidekick, but from those I've seen, only the magnet is installed inside. the reed switch is on the outside of the mast.
You didn't ask, but....
The motor lead choke (you'll also need on for the reed switch) that came with the Sidekick is wholly inadequate. It measures about 150 ohms at 10 MHz. The choke needs to be about 10,000 ohms at 10 MHz.
If you go to my web site, and look under "Antenna Controllers", you'll find the information you need.