Can an XM radio FM Modulator be put in the Shaker systems???

Yes, if you're running an FM modulator, you can pick up your XM radio from any FM radio that will tune to the frequency that the modulator is putting out, including the Shaker systems in the '05 Mustangs. Note that you'll probably have to have the modulator close to the antenna wire somewhere as the modulators are required by FCC regulations to be low powered so as to not interfere with anyone else's listening pleasure. Tune the modulator to a station that is unused in your area and you should be just fine.

If you take it on a trip, however, you'll probably find yourself retuning the modulator often as you cross into areas that use the frequency that you have the modulator set to... ran into that running an MP3 player into a moving truck via FM modulator from PA to FL...

JS