Hi Ben, you may need to change the resister control pack for the heater blower if it is burned out, the air con will not work, Its easy to change, its in the side of the heater (passenger side) will cost you about £25 recommend getting it from a ford outlet.
Just checked in the book again the locking unit as small plug, so this means that the earth that you need to check is around the rear lights. that's not as bad should be no panels to move, just carpets by the lights.
The main earth point you are looking for is in the tailgate, by the locking unit, you do need to take off some panels to get to it. if that does not fix it, disconnect the second light unit that you have fitted to see if it makes any difference. and did you use a stand alone earth for the second light unit..
you need to get the right one, one with small badge(ford), or one with big badge(ford) they are not swappable, you can't put big on small, different plugs and the steering wheel is different, it looks the same but its not.
had to have my sons car done, parts around £24 and fitting was about £80, did not have to take arms off, would have done it my self if a had the special puller, cost to much to buy puller that you need to do the job..
Hi Pete, Have you checked the tyre pressures. also I am not to sure if the mondeo uses the drive belt or an electric motor to work the steering pump, if it is the belt driving the pump it could be sliping at low revs giving you reduced power to the rack until you speed up.. if its electric it could need changing