If you purchase a dedicated VHF antenna and put it on the stern rail you'll get good AIS coverage for about 15 miles as the transmitting antenna is going to be high on the ships. The setup will cost about 50.00 and you don't have to think about the switcher problems as there won't be one.
Putting the antenna on the stern rail shouldn't be a 'major undertaking'. You don't want to put it on the mast anyway as it will be near the radio antenna which is not good.