TBs has already hosted two stories about how to connect an IR-Receiver to a standard 9V LEGO battery box. The trouble with the IR-Receiver is that it doesn't draw its current from the same wires as the motor and so the extension/conversion wire doesn't work in this case.
The first solution involved using a PF battery box (without batteries) :
The second solution consisted of using small strips of aluminium foil to short C1 and the 0V line as well as C2 and the 9V line :
Today I tried my hand at a solution proposed by Rob Hendrix (Brickmodder) in BrickJournal 7.
The solution consists of prying the light grey connector of the conversion cable open (the one that is top PF and bottom 9V) and shorting out 0V with C1 and 9V with C2 from the inside:
Make sure to carefully push the bent black wires between the metal forks that are the bottom part of the connectors. Then place the 9V bottom part on top and press hard to close. Check to see if all the connections are good by using your modified connector to hook an IR Receiver up to a 9V power source. Disconnect and reverse polarity immediately if the IR-Receiver's little green light doesn't go on, as you might damage it if exposed for too long to the wrong polarity.