[ricm]

no problem.

Once last thing to be careful about - check the polarity of the signal coming out of the MFU. I -think- that the MFU provides a common +ve supply to all the LEDs, then switches them on the -ve side down to GND. Whichever way round it is, you need to make sure that your opto-coupler outputs work in the same way if you want all your trailers to be interchangeable between each other and the truck. The opto-couplers can equally switch a +ve supply to the LED, or they can switch the -ve side of the LED down to GND as long as you wire it up right for the operating mode you choose.