More updates. The A75 has a setting to enable the internal GPS. This was OFF. I haven't turned it off, so I assume it was off off before.
If I switch it on, I'm getting the GPS position displayed on the plotter. Some progress. Still, I'm assuming an external GPS antenna was connected and still is, but does not work as expected.
However, going into the plotter's setup and making it search for GPS sources, I'm getting a popup about some devices blocking the "orchestration" (see photo). The popup says, this was not an error - but what else?
Furthermore, if I try to setup the P70 in the cockpit, it's crashing. It's always showing "no pilot". If I enter the setup and make it search for sources it will reboot.
The i70 will show depth information sporadically. The error about "No GPS fix" is gone, as long as the internal GPS of the plotter is enabled. Trying to configure a different GPS source on. the i70 tells me that it is "impossible". At least, it's not crashing like the p70.
I'm waiting for the electrician who reconnected the wiring. Hoping he will find the issue.