Radio Recption

Hi, got an odd problem here.

Polo 6n2 1.4tdi 2000

JVC KW-R400

Got the radio above, and came across a forum saying that with an aftermarket radio you need a aerial booster, so i got one and fitted it. It worked great for a week or so, until driving down the motorway it just died. Finally found a leak around the antenna base, so got a shark fin one, tried that and it had been hit and miss, sometimes decent others not, but i cant always get the same station in the same location, so it just seems odd. Anyway i thought it was the booster, so i got another, but no change. What i did notice is that when fitting it, upon moving the aerial around, it got better and worse, but not in any direction. I also noticed that it worked better without the aerial plugged in (when i wasnt getting any reception with the aerial in).

So is it the shark fin aerial having bad reception, the aerial cable being broken, or something else?

I'm ready to sell the aerial and buy a standard base. I also still have the original base in the boot which I'm going to try to see if its any better, but then, why did it cut out all together with that one?

I dont want to have to put a new aerial cable in as it will be a bit of a pig to go over the headlining.