Tamiya box trailer side marker led kit ?

I don't believe there is a specific kit to accomplish what you need. If you want to sacrifice your trailer brake and reverse lights, then your new lights will only work when braking or reversing. It is best to use an output with constant voltage I.e running light to power then all the time. For the lighting you will need to buy multiple LEDs and wire them in a series loop to sufficiently power them. Not sure what voltage the power legs output, I recommend checking them with a voltmeter. There is a sticky in the electronic tech part of this forum entitled all about LEDs. I highly recommend you read it, as I learned quite a bit myself reading it.