Apologies to all for being techy, but some years ago in Austin Rover days, all the cars were subjected to a scanned frequency in a chamber with a field strength of 50 V/m at the car, particularly concentrating on likely use of on board transmitters from amateurs and emergency services etc. The electrical system had to withstand this without malfunction from kHz to Ghz. The test transmitter power to achieve this 50 V/m was quite often in several hundred watts approx 2-3m from the car. Typically the response of the car was very subject to the resonant frequencies of the various harnesses, so sometimes the Tx power could be much smaller. If you can translate your transmit power into a field strength at the car harness, then that will give you an idea. However I don't have an up to date EMC standard for the field strength now used, but MG would have to gain EMC compatibility to a national standard to gain homologation for the car to sell in the country you are resident.
Frankly, with your 5W or 10W Tx, it's very unlikely it would affect the car unless you were operating at frequencies exactly on resonance of the harness (i.e. the resonant length of the car). That would be quite a low frequency probably around 100MHz.
Sorry, I did apologise....
