Thanks a lot for your comments guys, it make my day!!!
I've never seen that kind of trailer (here we call it a "rack & tarp") on other forum too but I'm surprise to hear it from you Paul (Tamiya cowboy) because I know that you're a real expert and very active builder, present and renowned on many forums! It's a kind of guarantee!
I already know SS-Tronix and his system is really impressive. I asked for a submission some months ago but wasn't ready at that time to spend so much money. But more I think about it, more it seems the best solution! I'll probably contact him for a complete submission soon.
For now, I'll try to connect roof lamps on the four outputs of the mfu planned for the speed indicator (3) and the original roof lamps. But I didn't make any test until now to see if these outputs can take four leds... I also ordered a relay for making a separate circuit for lightning the truck if it doens't work directly by the mfu, but i'll receive it only in ten days.
I'll keep you informed.
Gaetan