Hi, This quite simple actually. Though if you check her car battery it is most likely anywhere from 12v up to around 15v. You need to find out what the amperage draw is of the transmitter should be really low in the mA. To find resistance value- (Rs = Vin - Vout / I) Where V-in is your voltage from your battery, we already know that the transmitter is 9v so thats going to be around 3 to 4 divide that by your amp draw (I) ...i.e 0.04 is 40mA, to get your resistance. Then go buy a 1/4 watt resistor in the nearest value, go larger rather then smaller add in series with your positive voltage and that should work. Good luck
Mxitman :wink: