It is not necessary to route wires from under the seat -- all connections can be made inside the headlight shell. After all, there is already a constant hot high current wire and a switched wire at the ignition switch, so why not use them? A generalized wiring diagram for switching:
Note the constant power wire is connected to the normally open terminal, not the common: this prevents the normally closed terminal from being powered up when the relay is off.
And a method for connecting to the wires without having to cut and splice, outlined in the 599 headlight thread post 86: https://www.wristtwisters.com/forums/...ht-8592-3.html
I recommend a sealed fuse holder, miniature toggle switch, and switch boot for protection from rain or washing. From McMaster Carr:
-- SPDT miniature toggle switch P/N: 7347K75 $7.02
-- Toggle switch boot: Specify black, gray, red, or white. P/N: 70205K3 $5.68
-- Fuse holder: Sealed Mini fuse holder, P/N: 8110K1 $3.16
Mount it where you can get at it without having to open the headlight shell.
-- Relay: 12V DC coil, 40 Amp contacts. Dimensions: 1" 1" 0.9" P/N: 9672K29 $7.89.
This is a standard Bosch type relay available from any auto parts store. When mounting be sure the terminals are pointing downward to prevent water from pooling around the terminals and / or entering the case.