Just a thought, and excuse me for stating something so obvious, but is the aerial earthed correctly, given that it will need to be connected to the chassis by a bit of suitable braid on a GRP shell?
Not the easiest of locations to get good reception on AM, a grp-bodied car. I know, I just don't bother trying to use the AM bands in my Rebel and that has a screening mesh laid into the roof panel. Don't bother doing that though, it isn't worth the resin down the neck for all the good it does!
