Possible Display Out Problem...Sound works then dosen't.....

[crazant]

1st post so please be gentle....

I have puchased a cable that goes from ipod dock to RCA ( Red & White) so i can use a mini amp in the back of my van to play music.

I have tested the cable on my home TV and it works but if i unplug the dock and plug it back in it only plays sound out of the iphone and the only way to get it to work again on the TV is to shut phone off and reboot..It won't do it off a respring..

Last night i went to my van plugged it into the amp and hey presto it worked but after unplugging the dock and retrying it will only play through iphone itself....

I don't change anything between the unplugs so its got me lost...any ideas

This is my second cable and it does the same on both and when it works it works fine

This is the cable :
http://www.ebay.co.uk/itm/IPOD-IPHON...item3cb3336c16

Please Help

Thanks a lot
Ant