Pairing an FM transmitter to a car radio shouldn't be difficult. You simply need to find an unused frequency that both units can work with. Many transmitters and radio have 87.0 MHz as the lowest frequency available. As there are no UK broadcast stations below 87.7 MHz, choosing a frequency such as 87.3 MHz should be fine. (If you need to find another frequency that's not used by broadcasters in your area, see the list here:
http://www.frequencyfinder.org.uk/FM_Stations.pdf )
Once you've chosen a frequency, set the transmitter to it and provide it with an input signal. (i.e. connect your phone to it). Then use the 'scan' facility on your radio to find that frequency. When you've found it, use the 'memory' facility to store the frequency as a channel that you can select with just one press of a button.