I'm assuming the garmin is outputting true TTL-Level voltages?? (tested at +/- 2vdc)... This can't be healthy?
True.Are you sure you wired those converters the right way round?Normal TTL voltages are 0 to 5V but you need a 0 to 3V3 converter on the Zero if they are being fed into normal pins.
No other issues other than when mains power to arduino is off, the power led's have a dim and fast-pulsed look.
My friend is worried that if for some reason the gps is outputting 12V+ and my meter is too slow to see it?
Never connect an input to an Arduino, or any other piece of electronics with the power off. This causes parasitic powering through the ESD protection diodes and can damage things. This is what you are seeing with the LED.
The golden rule is not to exceed 0.5V above the voltage on the power pin of a chip. If the chip has no power this means don't exceed 0.5V. For a chip powered by 3V3 that means don't exceed 3V8.
we have just ground and tx-out from gps going with bare wire into the db9 connector on pin5 and 3. Data receives fine.