Hopefully i can help..
You say radiator i assume you mean you had the heater matrix was replaced - this would most likey cause coolant to form or leak out into the footwells over time the coolant would soak into the carpet or underlay and depending how much was in the foot well may of traveled up to the rear footwells..If you positive the coolant level isnt dropping and the front footwells havent become wet again .then as you mention or describe this could be a factor from the the initial problem you had and hopefully rectified properly..all you can do really at this point is to dry out those footwells in the back which will require what you may have done before in the front - that having to lift the carpets and dry it out best you can..or maybe try an autovaleters who may have a wet/dry shop vac or even household or industrial carpet cleaner to suck the water/coolant out - although the sponge underlay will need more drying out or removed as it will have soaked up the bulk of the coolant..either cut it out the wet parts and remove it and dry it out away from the car or place absorbent towels anything that can absorbe the coolant underneath..I do hope this helps..if you need to ask anything else or include anything further then just continue..
thanks for your reply. As you suggested - we will try to dry out the patch. It is pretty wet. Up to this morning the coolant level hasn't dropped (we have drawn a line to show the level. hanks for your help.