ive got a similar problem but with AM only.... with the clarion it dowsnt work with a CCR600 or anyother standard radio it does :-?
Very common fault with aftermarket radios, althoug the reasoning behind it makes no sense.
With the standard radio, the tuner is very very poor quality and as such will pick up just about radio station with a bit of wet string attatched to theh rear, as it is not to critical about the EXACT frequency it is looking for.
With an aftermarket, good quality tuner in your car, you now have to be a lot fussier over the grade of cabling and aerials fitted as the tuner is actually now looking for a more precise signal to be received. This is very noticeable on FM, and the improvements are often very obvious. However, when it comes to AM, there are so many electronics inside the car emitting their own noises and frequencies, that it is not uncommon for some of the harmonics to actually partially block out the AM stations. For this reason, you may have noticed that a lot of the more modern and better quality aftermarket car stereos actually have removed MW as an option completely, that and along with the fact that apart from the cricket and Five Live, everything else can be found on FM or the preferred DAB.
Its just a bit ironic, and no more so for us selling them, that if you want to listen to five live, then the expensive aftermarket stereo is going to perform a lot worse than your nasty OEM one.