I got one of these docks from Amazon:
Code:
http://www.mobilefun.co.uk/dual-desk-dock-for-htc-desire-z-p26896.htm
A few problems with it, firstly, the angle at which the phone sits when docked makes it of no real use as its simply too vertical.
Secondly, you have to really pay attention when placing the phone into the dock, the USB port sticking out could easily knacker the port in your phone if you're not careful to line it up and press exactly straight down (and be pretty firm with it too!); it almost never goes in perfectly the first try.
Finally, the secondary battery charging feature has stopped working for me too; meaning I now need to place the battery in the phone to charge it, I've yet to take a multi-meter to it.
Also, the blue light on the top has to be covered with blu-tac if you're placing this next to your bed, its just too bright and in my opinion not needed when the LED on the Desire-Z tells you if its charging or not.
This dock is more of a pain in the arse IMHO, I find myself using the cable it came with more and more now.