Problems charging iPhone 4S

[Den]

As some may have noticed one of the Apple haters has now got two iPads, two iPods and a iPhone 4S    I have run into a problem with the iPhone in that it will not charge in my van and I have read that you have to buy a 2.1amp rated charger for the car. I have a usb car adapter and I bought a double usb charger adapter from RadioShack but neither works. Any thoughts and do I have to buy a special car charger?   

[Simon]

I've got one of these which works fine, Den:

http://www.amazon.co.uk/dp/B00436GBSC/ref=cm_sw_r_udp_awd_rrxzsb1EN29FT

[Steve]

http://www.amazon.co.uk/gp/product/B004T1VSRK/ref=oh_details_o00_s00_i00?ie=UTF8&psc=1

As an adaptor should do the job

[Den]

The double usb plug I bought from Radio Shack has a rating of 3.1amps and still does not work 

[Steve]

Is that both sockets Den as the part I've got somehow it's said to have different ratings whether that's upper and lower/viceversa I'm not sure or there's a switch on the thing I've not found yet!