Is the GTM 20 the older style that looks like a cigar at the plug or the newer model with the receiver in the middle of the cord? If it is the old style, does the light that is closest to the car lighter flicker when you turn the adapter in the socket? I have found that if you unscrew the tip and use some emory paper or light sand paper to sand the ends of the fuse and the metal tip where it contacts the fuse, it can cure intermittent power issues, which will prevent your GTM 20 from being recognized. I have also seen inside the earlier GTM 20s where the plug to the circuit board inside starts to work its way loose creating either no power or intermittent power issues. It is fairly easy to open the receiver and reconnect the plug. It essentially pops apart once you remove the tip and the metal ring along with the metal contacts.