I've tried a number of extension cables from eBay all of which failed, the only one that worked was the CableJive
dockXtender: Dock Extender Cable for iPad, iPhone and iPod
Also have a look at one of my threads on how to route the cable out from the glovebox
dockXtender: Dock Extender Cable for iPad, iPhone and iPod
Also have a look at one of my threads on how to route the cable out from the glovebox