Finished the Garmin cable (had the male pins/female sockets in the wrong gender plugs). Setup and tested the GPS feed from my Garmin 296 to the ACK E-04 ELT; it works, but I had to go into the Advanced interface settings on the Garmin and turn off the proprietary protocol. Works with Garmin set to Aviation In/NMEA/VHF out, 9600 baud.
Finished attaching tail light/strobe to fiberglass tip with backing plate epoxied (structural epoxy T-88) to fiberglass, and tail light screwed to backing plate via nutplates; used Loctite RTV to seal everything. Still need to cut hole in fuse end and route cable.
