I have no knowledge of the "Battery Tender" brand, but a quick look at their web site
shows that the Battery Tenders have a two pin quick connect plug,
which then connects to different types of adapter, such as :
Fused ring terminals for direct battery connection,
Fused alligator clips for temporary connection, and
Fused Cigarette Adapter Cable Assembly, to plug into a "Cigarette lighter" socket.
All you would need to do would be to get any of those adapter cables ( the cheapest one ),
remove the end fitting(s), and replace with a Hella type plug,
which you would then plug into the permanently live socket by the riders left knee

"Hella" type sockets are widely available - where the wires are secured with screws,
so no soldering needed, and is a relatively easy "DIY" job

