RE: Simple GPS to PC idea
If your Tx's input consists of an eletret mic on a pigtail, then it will be a mV sort of deal. If it is one of the usual designs that did not ship with a mic, and is just a wire harness, then it will usually be a standard 1V pk-pk line input. They often internally AC coupled so you will probably be able to skip installing your own cap. If you do, be sure to plan for low freq reponse to support the raw (unmodulated) 1200 baud serial data. You do not need an optoisolator, but if that sort of interface is easier for you to design then go for it.