The ECU and both fobs on my 620ti are working perfectly but, needing to test a spare ECU and its matching Remote Fob, I set about the job in the following way. First, you need to remove the boot lining trim on the right hand side. The remote C/L ECU is located behind the boot lining here:~

http://img.photobucket.com/albums/v465/MGJohn/Tools%20and%20Methods/VariousThursday14thJuly2011003.jpg

Trim carefully pulled to one side to reveal and access the ECU :~

http://img.photobucket.com/albums/v465/MGJohn/Tools%20and%20Methods/VariousThursday14thJuly2011001.jpg

Piggy back ECU test.

http://img.photobucket.com/albums/v465/MGJohn/Tools%20and%20Methods/VariousThursday14thJuly2011002.jpg

It works ... ;)
.

If I had known about this a few years back I could have sorted my remote central locking ages ago!! Also seen a few tidy 600s with fobs in the scrappies and left the parts there!!

Still working fine touch wood (havn't got any door locks at the moment)