AIS receives well. No issues with VHF. I'm getting the dreaded red antenna error on the NAIS400. VSWR is 119:1. 

When I directly connect the existing mast head antenna cable to the NAIS400, bypassing the splitter entirely, the AIS works perfectly, with a 1:1 VSWR using aisPro2 software. My location immediately appears on Marine Traffic. Good GPS signal. No errors. So I believe I've isolated the issue to the splitter itself, or the cables between the AIS400 and the splitter, or the power supply. 

Replaced all cables - no joy. 

Had the old NSPL-400 tested, and while it bench tested operable, my tech noted that the circuit board showed damage, and he glued it back together, pronouncing it serviceable.

Not liking this scenario I purchased an updated NSPL-500 which I just installed - and lo and behold, the power supply light isn't coming on. Results same as above - receives AIS well, VHF functional, no transmission function, very high VSWR, red antenna error light on AIS400 . 

(The old NSPL-400 splitter has no power or fault indicator lights and it didn't occur to me to test power supply)

So, now I suspect the specified 1amp in line fuse in the 12v power supply to the splitter might have blown, or I have a power supply problem. 

The power supply cable for the splitter is combined with the FM antenna cable, and the cable disappears into the 6" thick tightly wound cable loom. Not friendly. 

Anyone have any idea where the inline fuse might be for the splitter, or where this AIS splitter gets power from? Have been unable to determine from my wire diagrams. 

Boat is a 2016 505. 

Not crazy about finding another power supply unless I have to. 

Thanks in advance to anyone who might have an answer, or who may have tackled this issue previously.  

After undoing a substantial amount of the cable bundle, I gave up the search for the fuse. 

Ran a new power line to the NAV switches on the Wurth panel, added a new in-line fuse, and all is good.