Many of the car adaptors for XM/Sirius radios and MP3 players appear to be capable of going down to 87.5 MHz. While there are FM broadcast stations as low as 87.5 MHz in other parts of the world, in the United States, the spectrum for TV channel 6 is 82-88 MHz. FCC Part 15 rules specifically prohibit any fundemental emmission in the TV spectrum.
This article in TV Technology discusses the issue.
A very small number of class D FM stations that are extremely distant to TV channel 6 stations are permitted to operate on 87.9 (also known as Channel 200). REC Network's website FMFIND.info does not consider 87.9 MHz as a possible frequency for consumer electronic devices.