External amplifiers won't solve the problem... assuming the system is working properly.
The problem with an external amp is twofold:
1. You can't amplify signals that don't exist coming from the antenna. The efficiency of reduced-size antennas is less than mast antennas. The signal to noise ratio is lower - you just don't have as much signal to work with.
2. You set yourself up for a boatload of problems with overload - inability of the receiver to handle strong signals. When you experience overload, you will definitely be unhappy with the system, especially when you get distortion and break-in from interfering stations (as opposed to desensitization)... it sounds really, really, bad.
Stay away from amplifiers. If you have a problem with sensitivity, find out why and fix it. Unless something is broken, weak signal complaints (ie: "stations are weaker than my trade-in") are almost always due to the poorer signal to noise ratio on a glass or micro-mast antenna vs a full size mast.
Yes, there are things that can focus your attention on the 'weak station' sound like not enough rolloff and blend (making it sound worse), but the OEMs spend a lot of time tuning these parameters, and a newer car will generally not have these issues.
Summary: Don't connect an amplifier to your antenna. You will not like it.