I would not use a plain interface cable.. there probably is some electronics in whatever cable you saw.. either that or the owner is an idiot.

the ODBII interface is car voltage.. 12-13v.. but the serial port is 5v.. atleast that's what I remember.. it's probably a good idea to use some kind of regulating adapter.. some ODBII cables i've seen have optical isolation.. also not a bad idea when hooking this stuff up.