Where you using an RF Remote where the TM751 has to receive the signals or a powerline controller like the cm15a sends line signals from the house to the garage? Two different ways to have a problem. One is an RF range to the TM751 the other a powerline loss from the house wiring to the garage.
Be very carefull with the TM751. It is not like the cm15a that has a power transformer in it for isolation. The TM751 has a PowerLine Derived power supply. Common on the PC board is the HOT AC from the wall socket
. Actually almost all of the X10 modules controllers and switches are AC Powerline Derived. Only transformer ones are those that a simple AC Derived Supply can't provide enough current or for safety. Like the TR16A Touch Tone Responder has a transformer and my guess is for FCC Phone Interface Acceptance
The antenna of the TM751 is isolated from the insides by a set of plates with the case in-between. Basically it acts like a small value capacitor. Couples the RF from the antenna to the circuits inside.
Now anything you do to the external antenna is isolated from the power. So anything connected or modified on the outside is safe. Anything inside is asking for a shock or sparks.