Several changes will be necessary before the practical use , because my circuit is an experiment stage. If you want to use only NMEA data, you can use this circuit. My experiment circuit is here . MAX3221 is powered by TX of Visor. The anticipation circuit diagram of the genuine cradle.
Circuit diagram is here .
Right angle arrangement antenna and automatic switching unit for GBR-21. Automatic switching circuit diagram is here . But it does not work. So, I removed all electronic parts and added MAR-6 amp. Circuit diagram is very simple.