[Classic30]

Also your unit has a built in Power Divider/Splitter for connecting to a VHF Radio and a single Antenna. I'm curious as to how well that functions.

I've had an MXA-5000 installed for a little while now (to a single mast-head antenna) and haven't yet noticed any issues with the VHF around reduced range, etc. It was pretty easy to set up and, following the instructions, worked straight out of the box. You need to be talking for a long time for it to lose the AIS targets.

Assuming the DB9 connector was wired according to the manual, I suspect the OP's issues will be related to his choice of USB adaptor and/or baud rate. I'm using OpenCPN which has an in-built serial data monitor, making it easy to check that the comms is working properly.