If your external antenna no issue and no wiring issue, and you are using an aftermarket headunit, then you can read on.

I tried going to a car audio shop and complain the same thing to them about my FM signal being weak.
One shop told me to install a FM booster, so I bought an FM booster.
It didn't improve...at least for me. RM60 including installation.
So sad with my wrong investment, I went to a few other shops and just talked to them while hunting for my subwoofer or amplifier (I haven't made up my mind which to buy that time).
Many say it depends.
Only one guy told me its useless. It really depends on your headunit integrated signals, meaning you need to change headunit. My problem also came when I install my aftermarket kenwood headunit.