The key fobs transmit a digital code that the onboard computer reads. the code is never the same twice in a row and changes according to a pseudo random scheme that is synched between the onboard computer and the fobs. Further, a large number of different pseudo random algorithms are used, so that if by chance one fob syncs with the wrong vehicle one time it won't do it a second time. Anything breaking that has either insider information or an extremely sophisticated computer program. I'd bet on insider information from someplace like onstar. Further a rheostat is just a variable resistor. Who would that change a transmitter frequency?