The UK cars side indicators are literally wired into the supply to main front indicators, so simply share the current. If you are getting fault light, check that the bulbs you are using are correct type, and in the right way round in the bulb holder. I think these are 10 watts. What have you done with the side marker bulb supply wires? The prob may be as you suggest to do with these. If you blank off the live and put the other to earth, may fix it.
Out of interest, the UK cars do not have the red illuminated rear markers, as standard we have plain amber reflectors. So I have fitted a set of USA red markers and have piggybacked the wires into my rear light wiring with no problems, and I have used both 5 and 10 watt 501 bulbs with no issues.