The easiest way , using your existing setup, would be to connect the wire going from your switch to the light, to pin 85 of a standard auto relay, and pin 86 to earth. Now when you operate your lights from your switch, you operate your relay instead.
Connect a full seperate +ve battery to terminal 30 via a fuse. Connect terminal 87 on the relay to the light.
5-pin-12V-automotive-relay.jpg
This is the easiest way given what you already have set up. After fitting this, instead of using your tail light +ve feed to operate your work light, you will be using that to operate the relay, which is not much load at all, and you will have a dedicated +ve to run your worklight. This will also prevent the work light blowing the tail light fuse and vice versa if there is ever a fault in the circuit. If there is a fault it will also make fault finding a little easier as you can see which fuse if blown.
cheers Tony.
BTW there are other ways of wiring a relay into the circuit.
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