That splice has a low probability of success. There are several light gauge wires int that cable... 4 or five probably, and while it might work initially, it won't be very durable. The only way it could be done is to cut off the Garmin connector and get a new set of pins for the Lowrance connector. Crimp the pins on the wires and then snap them into the connector housing. This requires the correct pins and the correct crimping tool. I am also assuming the Lowrance connector could be disassembled. Without going out to the boat I can't be certain. It may be a fully sealed connector... Actually, I think they are.
If your Garmin has a NMEA 2000 capability, you could
Eave it connected to the transducer, and network a second Lowrance unit to it. The Lowrance can get the sonar data from the network and you will have a second display for ur chart. If you boat configuration would benefit, you could move your older Garmin display to the aft where you can use it to watch you DR ball depth. It is very handy when setting gear.
Franko
MILF (Man, I Love Fishing)