Not had any issues myself except for when travelling, i am guessing you mean issues even when in a local CBD?
Firstly I get more or less the same reception regardless of my aerials position (up/down).
Have you ripped out the Headunit and double checked that the two plugs are still connected correctly, i found out one time that swapping them aint a good idea for reception.. they need labels as are same connector.
FM Amplifier, is easy to install "If" it has the right connectors of your cars aerial or there is a join/extension around the glove box area.. They are just an extension cable type with a grounding screw.. Only good for older cars where the mount was plastic and did not contact the body... BUT may help..
Digital radio receiver, not seen any decent ones for cars but then you would need to input it to the existing headunit.. so depending on what model unit it is, you may need todo the old ipod hack or go straight to the amps and so on.. I'd sus out the aerial issue first and see if it is a bad match to the car.
