The key is to understand that NMEA 0183 (NMEA 2000 is completely different and easier) is a "single talker, multiple listener" protocol. You can't put two "talkers" on the same circuit. Most APs and GPSs have two, a listener (data in) and a talker (data out)(and grounds for each circuit).
So the talker (data out) connects to the listener(s) (data in).
They also confuse the issue with + and -, hot and ground...... absolutely no standardization for naming this stuff among manufacturers.
In every CIRCUIT the grounds, (-)s, are connected together. And don't confuse the shield (protects the wires from interference, a chassis ground) with the circuit grounds.
So GPS talker circuit to AP listener (2 wires) and AP talker to GPS listener (2 wires)
Then the question becomes when I get a chart plotter/dept/speed/wind for Christmas which talker should that instrument listen to/talk on?
It can’t listen to both as that would connect two talkers on the same circuit….. which is why 0183 is being phased out.
This may not sound like much of an issue now but you might want to consider where you do you splicing so the chart plotter wiring is less onerous later.
For the record SeaTalk is not going to work with anything that is not RayMarine.