Car FOBs/jammers are on several different frequencies, 315 Mhz, 433 Mhz..and maybe more.
DX used to sell them (called "modules" now..without the case, board only).
The old HP sig gen tuned to 315Mhz, 1khz modulation & roof ant pretty much shuts down the GF from using her FOB ( and the garage door opener)..A temporary thing.
I get in shit for it every time! ;P
{edit}
Some might use a VCO (cheaper by the dozen) on eBay, tune it to the center frequency {it drifts}, one might add an oscillator input and a chip RF amp.
This is all very illegal, and anyone wth a directional antenna and spectrum analyzer might find you.
Shh!