I have the same model 9000XS although mine is H reg. I had exactly the same problem to solve before christmas. If your heater is the same as mine, you may find that the resistor back unit consists of different wires, rather than a set of nicely wrapped up resistors. When I took the unit out I found one of the wires was broken. Whether this was my clusminess at taking the unit out or it had gone by itself I am not sure.
Rather than try to find out what the wires were to replace them I decided to go for a replacement unit instead. This comes with "real" resistors etc, nicely wrapped up and difficult to damge. I bought the part from eurocarparts for about £50. It is easy to replace 30mins - 1hour depending on how firmly seated the false bulkhead is.
Try checking the threads from early december type time, because someone posted some good pictures of the part and where to find it