[ricm]

if you want to add extra LEDs in parallel, I would suggest buying 4x LEDs and replace the original Tamiya ones too. The reason being that unless you can find an identical spec (and I don't know what Tamiya's spec is), you're more than likely end up with one pair of LEDs brighter than the other pair as a result of even small differences in current drain/ voltage requirements between the original and new LEDs.

Secondly, if you are using MFU as power source it is a bit risky since the extra current drain can pop the MFU. I've known people run a second trailer light set in parallel from the MFU for a road train without trouble (initially, at least).....I've also known people break the MFU trying to add extra head lights which is kinda the same thing. Safest option is to use the MFU channel to switch an opto-coupler, you can use its output to drive your 4x LEDs. The opto-coupler isolates the MFU from the extra current draw for the LEDs. If you search '4N25' or 'opto-coupler' on this forum you should find a few threads on how to do this. Not expensive to do, but you'll need a soldering iron, a bit of veroboard or stripboard, and a power supply on your trailer (which you could take from the truck itself).

If you're not using MFU, then you should be fine to just add the extra LEDs as long as you use the correct protective resistor....you'll need to work out the resistance value based on the LED voltage and current, and the supply voltage.

Or you could leave the trailer light kit as it is and use a spare 'aux lights' plug on the MFU for the extra 2x LEDs?