Bad FM reception

Recently fitted Pioneer DEH-4400BT radio in Broadway El Duo. It has external antenna mounted almost horizontally above the passenger door. I get a lot of interference on the radio in areas where I know from my other cars should be ok. The original radio was not great but the new one is worse. Has anyone else got this problem and have they solved it.

My experience with bad FM reception has ALWAYS been a poor/bad antenna, ensure radio is earthed, a good connection of the centre copper wire to
the centre pin on the co-axial plug & to the centre screw on the aeriel mount, then ensure a good conection of the earth-copper sheath to the outter clamp
on the plug & again the aerial.
If the antenna is bolted to metalwork, ensure the paint is scraped off to bright metal on the underside where the nut/washer/earth braid is connected.

Most modern FM units are very sensitive & tolerant of vertical/horizontal orinentation, but you can try & alter it a bit.
Also ensure the 'whip' is properly fitted to the mount.
Although not really nesessary if mounted to GRP, a separate earth wire on the mount can help.

Also, is the aerial the original and the correct length. FM reception is very dependant on correct aerial length - as I found out when the tip of my Seat Ibeza aerial broke off. It's to do with wavelength apparently and "modern" aerials have the antennae wire spiralling around the stick.